[NEEDS SAVES] Partnered Sims turn into siblings/family relationships

I read all the related bug reports until late last night.

I was disappointed to learn that the first report of this bug was in 2019.

This bug has not been resolved for several years.

There is speculation that CC may be the problem, but I have never used CC at all.

All I used was the appearance cheat.

Anyway, after a few tries, I figured out how to avoid this bug.

However, the family tree that was already broken could not be repaired.

And I guessed under what circumstances the bug would occur.

I'm uploading my game situation in the hope that it can help fix the bug.

1.family member

  -  A(Female, second generation, B's wife, both parents present in the family tree.)

  -  B(Male, random NPC sim, A's husband, no parents in family tree. Used Appearance Cheat.)

  - child 1(A's nephew, Adopted as B's child. A and B appear as parents in the family tree.)

  - child 2(A's nephew, Adopted as B's child. A and B appear as parents in the family tree.)

  - infant1(A's nephew, with out B's family tree )

  - infant2(A and B's children)

2. Situation before bug occurrence

  - A family of five was living together in one house. This is before A and B’s children are born.

  -  At this point Two children when used Plan outfits and no bugs occurred.

  - B adopted A's two nephew.

  - A is pregnant.

  - A gave birth to a baby.

3. Things I checked after the bug occurred

  - After A gave birth to her baby, she used the plan outfits menu.

  - The family tree is broken. A and B are a married couple, but at the same time they are sibling.

  - I confirmed that using CAS caused a bug.

  - Since bugs occur when using CAS, creating a new file does not solve the problem.

  - Even if I use the Manage Worlds menu and place it in a new file, the problem remains. When saving the family to the library, it is obviously normal, but when placing it, an error occurs. On their profiles, they are recorded as siblings and lovers. In CAS, their relationship is recorded as a married couple. It's a complete mess.

  - Bugs do not occur when you normally move to new house from an existing file.

  - Bugs do not occur when moving to new house from an existing file using the Manage Worlds menu.

4. How to avoid bugs

  - This method should be tried after you realize that a bug is occurring and before your family tree is ruined.

  - Choose one of the husband and wife and send them to another house.

  - The sim you send must be a random sim without family tree. For example, in my case, I have to move B, not A. This is because bugs occur when random Sims use CAS with their families.

  - After move out B, CAS is using when there are no random sims among family members.

  - Bring a random sim(Wife or husband) back home.

  - This way their relationship will not change.

  - However, even if alone ramdom sim, should not use CAS. Random Sims cause relationship bugs even when using CAS own alone.

  - Through this process, I learned that random NPC as spouses cause bugs. Sadly, random NPC Sims cannot have new outfits throughout their lives. But, only this way is protect lover relationship.

5. What is assumed to affect the occurrence of bugs

  - Random NPC sims family tree. B has no parents, but he recognizes his wife, A's parents, as his parents. 

  - I tried to add the parents to B's family tree but failed. cheats not working.

  - Other bug reports said that cloning using science and magic seemed to be the cause of the bug, but I have never used it.

  - Another bug report said that this bug occurs in same-sex couples, but this was not the case in my case.

  - I agree with the opinion that this bug occurs between second generation sims with parents and sims without parents.

  - I guess is that A's pregnancy and birth baby had no effect on the family tree error. This is because the same bug occurred in other save files When A is in the third trimester of pregnancy.

  - I'm not sure if the bug occurred after B adopted A's nephew. This is because B did not use CAS immediately after adopting the child.

  - B adopted a child, A gave birth, and there was an update 20240306. Afterwards, I noticed that a bug had occurred. I don't know which of the three influenced it.

- What is certain is that Random NPC Sims are the problem. It seems that the NPC whose parents are identified in the family tree may be different. This part needs to be confirmed.

6. Actions to take next

  - When the currently playing generation needs to change outfits, they send B to another house.

  - B does not use CAS on his own or receive clothing recommendations from a Sim with a stylist job. To prevent the use of CAS.

  - When the next generation gets married, they will not marry a Random NPC Sim. Sims will marry a sim who has both parents in their family tree.

Wow! I found out something new.

Their family relationships changed after the Plan Outfit interaction.

If you cancel the Plan Outfit menu, no problem will occur.

The problem occurs when you press the play button in the Plan Outfit menu.

They are a couple. However, they became brothers.

The person who used the Plan Outfit menu does not change.

The person who did not use the Plan Outfit menu turned into a brother.

This family relationship appears as a couple in the CAS window.

However, the profile states that they are brothers.

Even if you move a family to a new file, the same problem occurs because you place it by pressing the play button in the CAS window.

I don't know why this problem occurs.

Is the family tree the problem?

@milkyowl Can you duplicate this on another couple ? Is it both Sims having the issue or only the one you take to CAS ?

I tested this on my save and couldn't replicate.

Are you getting any lastException files ?

Product: The Sims 4
Platform:PC
Which language are you playing the game in? 한국어
How often does the bug occur? Every time (100%)
What is your current game version number? 1.105.345.1020
What expansions, game packs, and stuff packs do you have installed? EP: All expansions except For rent/GP: All game packs except For Star Wars/SP: All stuff pack/Kit: Courtyard Oasis Kit, Blooming Room Kit, Desert Luxe Kit, Industrial Loft Kit
Steps: How can we find the bug ourselves? Using the "Plan Outfit" interaction
What happens when the bug occurs? After “Plan Outfit” interaction, The couple relationship changes to Brothers and sisters
What do you expect to see? Relationships have changed in the profile and family tree. They are a married couple, but they are also brothers and sisters. Because of this The affection expression menu and flirtation menu disappear. I Guess, Because of this The affection expression menu and flirtation menu disappear.
Have you installed any customization with the game, e.g. Custom Content or Mods? Never used.
Did this issue appear after a specific patch or change you made to your system? Yes
Please describe the patch or change you made. 20240306 update

‌

After “Plan Outfit” interaction, The affection expression menu and flirtation menu disappear.

before “Plan Outfit” interaction. You can see three pie menu. Everything is normal.

After “Plan Outfit” interaction. You can only see massage in the physiacl intimacy menu. And serenade song menu.

--------additional information----------

After checking the answers, I confirmed the issue of changing family relationships.

So I modified the title and content.

New sims without a family tree did not have this issue.

I don't know if anyone else is experiencing the same problem.

I tried this and it worked temporarily.

Eventually, Bella Goth became my sims mother again.

Thinking the issue was that somehow the Sim was connected to Mortimer (who is already culled)

I deleted the affected the sims, created new ones, and at some point re-establishing relationships, Bella became mom again... Not sure what the issue is.

The only thing that worked for me, won't be a solution that is ok for most people.
I "played with genetics" made a twin of the wife, then deleted the wife. 
made the "twin" a wife of the husband.

so she looks the same but obviously is like a new sim. *cries*

Hi, I recently started having this issue.

Product: The Sims 4 (Digital Deluxe)
Platform: Mac
Which language are you playing the game in? English
How often does the bug occur? Every time
What is your current game version number? 1.105.332.1220

What expansions, game packs, and stuff packs do you have installed?

Expansion Packs: City Living, Get to Work, Get Together, Cottage Living, Seasons, Cats and Dogs, Discover University, Eco Lifestyle, For Rent, Get Famous, Growing Together, High School Years, Island Living, Snowy Escape

Game Packs: Dine Out, Outdoor Retreat, Spa Day, Dream Home Decorator, Parenthood (Every Day Sims Bundle), Parenthood (Pet Lovers Bundle), Jungle Adventure, My Wedding Stories, Realm of Magic, Star Wars Journey to Batuu, Strangerville, Vampires

Stuff Packs: Bowling Night, Luxury Party, Movie Hangout, Tiny Living, Laundry Day, My First Pet, Backyard, Cool Kitchen, Fitness, Kids Room, Laundry Day, Movie Hangout, Perfect Patio, Romantic Garden, Toddler

Add-Ons: Street Eats Digital Content, Awesome Animal Hats Digital Content, Life of the Party Digital Content, Holiday Celebration Pack, Up All Night Digital Content, Bust the Dust Kit, Country Kitchen Kit, Courtyard Oasis Kit, Desert Luxe Kit, Industrial Loft Kit

Steps: How can we find the bug ourselves? 

1. Create two same-sex sims in two separate households - sim A to have one parent and one twin with ability to get sims pregnant, and sim B to have two parents. Play game and gain high friendship and high romantic relationship - become girlfriends, become fiancees, move sim B into sim A household, move sims A and B out together, become wives.

2. Sim B to have science baby with sim A using sim A's twin sister as donor.

3. Sim A to adopt child as care dependent, then clear relationship between child and sim A's twin sister (as twin sister is mother of child - is this a bug???).

4.. Play game, save game, close game, re-open game and resume in sim A's parent household (game closed in sim A and B household - is this also a bug?). Sim A to arrive shortly without being invited.

5. Sim A's mum to give sim A's twin sister a makeover opening CAS (cas.fulleditmode not on) as work from home task while in Stylist career and while sim A is still present on lot.

6. After makeover complete, play as sim A and return to sim A's lot. Bug to occur from this point, when CAS entered with or without cas.fulleditmode on, and when relationships fixed, game saved, then reopened

What happens when the bug occurs? Sim A and B become sisters, grandparent/granddaughters, or aunt/niece
What do you expect to see? Sim A and B related through marriage only with child as daughter of both 
Have you installed any customization with the game, e.g. Custom Content or Mods? Yes, all mods always updated to most recent version after patch updates and before starting game, problematic custom content always removed before reloading game and without saving game
Did this issue appear after a specific patch or change you made to your system? After most recent patch and once makeover completed

Most recent save: Slot_00000008.save.txt

At some point, the siblings relationship was added to the couple sims .( I will call the man of this couple ‘D’ and call the woman of this couple ‘L’) I guess this error occurred cus I added L to D’s family and delete L’s family relationship with D’s other family a few times.Even if I deleted the sibling relationship using mccc, the sibling relationship is added again after editing sim or managing the village.(I was really noob so I didnt know how to solve error.)So I clicked the new game button after saving the couple in the library. And Its been 2month that I started new game. But I miss the other family(D’s parents and siblings.).but I deleted D’s parents and sinlings in library since If i add D’s parents and siblings to the game and make them to be a family with D , the sibling error occurs again.Im thinking of recovering D’s parents and siblings . So I’m going to copy old save files and put them in the save folder after deleting my current existing save files, then save only that family in the library and delete the old save file again and fill save folder with my current save file. Is this a safe way ? If I do that , Can the sibling error occur again?