Partnered Sims turn into siblings

This literally happened to me this morning. I played on PC and my legacy sim’s romantic partners are now their sister and brother. The only mod i have is MCCC. I’m shocked that there’s no fix for this, as when I looked it up, people haven’t been able to find a fix for over 10 years now.

My sims have 3 children together and at some point between the last one being conceived and being born, "brothers" was added to their relationships list. When the child was born, the birthing parent got a milestone for having a child descended from grim (not true), and the child's relationship to the other parent is fixed as nephew and uncle. 

The sims still consider themselves married, but cannot do romance interactions. 

I have no mods installed, and every pack. A screenshot of the relationship profile here: 

While the other sim was pregnant, he did get and use the skill mastery perk "skillful sleep". I am wondering if turning into a ghost briefly has something to do with this, because as far as I know ghosts can't have children. 

Here is a link to a gdrive copy of the save in case that is useful. 

reposting as requested by Crinict 

My sims have 3 children together and at some point between the last one being conceived and being born, "brothers" was added to their relationships list. When the child was born, the birthing parent got a milestone for having a child descended from grim (not true), and the child's relationship to the other parent is fixed as nephew and uncle. 

The sims still consider themselves married, but cannot do romance interactions. 

I have no mods installed, and every pack

While the other sim was pregnant, he did get and use the skill mastery perk "skillful sleep". I am wondering if turning into a ghost briefly has something to do with this, because as far as I know ghosts can't have children. 

I noticed my sims wouldn't romance each other anymore and went to the info panel to see that they are marked as brothers now....  but still also tagged as married in the info screen and in CAS.

One of the sims came from the gallery from cupids corner and I've also been having a visual glitch with him where they keep reverting to their original outfit after the makeover I gave them after each load screen but in CAS it's always the outfit I made for him.

The only cheats I've used in this save is cas. fulleditmode  (They had a twins and I could handle it so I deleted one of babies...)

I'm having this problem ( spouses becoming brothers) and my sim also tried doing skillful sleep! I'm not sure if it happened immediately after but it was def not before

So i play on Playstation 5. 
My two female sims were together for about two weeks and got happily married. Then after some time they got a daughter together through sciencebaby option. Suddenly after she was born none of them could do any romantic interactions with eachother. Upon looking at relationships gave me a huge shock. They were suddenly sisters…..

I tried moving them out seperatly and divorcing them. Nothing helps to remove it. I have never used cheats and never will either. This is a obvious very serious bug/glitch that is completely ruining my game. Any help on What to do would be appriciated.

Reposting here at EA request…

Married couple suddenly becomes sisters…

So i play on Playstation 5. 
My two female sims were together for about two weeks and got happily married. Then after some time they got a daughter together through sciencebaby option. Suddenly after she was born none of them could do any romantic interactions with eachother. Upon looking at relationships gave me a huge shock. They were suddenly sisters…..

I tried moving them out seperatly and divorcing them. Nothing helps to remove it. I have never used cheats and never will either. This is a obvious very serious bug/glitch that is completely ruining my game. Any help on What to do would be appriciated.

Same, it happened after my sims tried skillful sleep for a while, and I used cas.fulleditmode for a couple times then my sims becomes brothers with his spouse. I really wish they'll fix this bug cuz I  put so much time and affort with this household. T0T

My sims married couple has changed to siblings and still being married. They are brothers and husbands at the same time, but can't interact romantically.

I have the impression that the problem started after I changed their baby's clothes (by clicking on the changing table)

I'm not using any mods.

Same here no clue what is going on, but it's REALLLLLLY putting a damper on gameplay. Playing on Xbox so definitely no mods. Heeeeellllllpppppp