Partnered Sims turn into siblings

My married sims became siblings and now I cannot perform romantic acts with them. Also, the family tree shows them as both married and siblings.

This is happening to me, too. My main Sim thinks her husband is her brother and one of my other Sims thinks her ex-husband whom she has a child with is her brother. Specifically, this happens whenever I go into CAS. It doesn't matter if I make any changes or not. Simply loading into CAS breaks the family tree in this way.

Both couples did live in the same household as kids/teens but were definitely never related through blood, marriage, or adoption. I figured it was that dumb Super Siblings relationship bit the Parenthood pack applies to all underage Sims in a household regardless of their relation to each other but none of the affected Sims have that relationship bit with each other so I guess that's not it.

This initially happened to me in a modded game, so naturally, my first course of action was to figure out which mod was causing the issue. But after repairing all the relationships, I took out my Mods folder completely and deleted all my cache files, then reloaded the game. After making sure the family ties I had repaired were still intact, I went into CAS and exited to test in a vanilla game. The same thing still happened. Two of my Sim couples went right back to being siblings in a completely unmodded game with a fresh cache.

I’m absolutely devastated as my play-through of my real-life has been absolutely derailed, as now my spouse and I have somehow turned into “sisters”. I just can’t believe that this is happening in 2025 and isn’t being updated in the 150+ bug patch. I read on the forums that it’s been happening for at least three years, regardless of mods (I even just saw one from 5 years ago) 🙃

Does anyone know how to actually fix it? I tried cas.fulleditmode and it didn’t work (with/without mods). I tried MCCC, and UI cheats to delete the “sisters” relationship and add the “spouse” one. Both of our sims were happy because they were “newly married” and I thought “yay, back to normal”. Oh boy, I was wrong. Still can’t do any romantic interactions besides offering massages. I’m actually so sad about this, as it’s been my favorite save since I started it over a year ago as it’s my partner and I. I also have our best friends and cats in the save. 

All I want is to be romantic with my spouse (and hopefully have a science baby again one day)😢 since it’s my partner and I’s life but in the sims, I’m so so sad. I truly can’t believe this has happened, and nothing I’ve tried has worked to allow us to be romantic again. Please please help! I solely play as lesbian sims so this is so frustrating and actually game-breaking for me.

(I also read the only way to fix it is by adding **bleep** and allowing romance between siblings (ICK!). Literally so disgusting that this is the only option that might work)

hi! adding parents to both of your sims might work, for me it’s actually the only thing that helped to fix that bug. i had to use mccc for this — basically you need to create 4 sims and add 2 of them as parents to each spouse, removing the “siblings” relationships between them before that. i personally had to use mccc because the game wouldn’t allow me to do that in cas.

unfortunately, you have to keep this 4 sims in your game even if you don’t need them for gameplay, but this is the only workaround i’ve found (except for the mods you’ve mentioned) after hours of trying to fix my gay couple i’ve put my heart and soul into.

My spouses keep turning into siblings AND step-sisters, and when I remove it with cheats there are no romance options available besides "offer a massage". 

I removed all mods, repaired my game, and tried recovering my save, to no avail. Help!!

my sim and their partner were married and randomly they couldn’t romantically interact (other than offer a massage) and when i checked their family tree it showed them married to their spouse but it also showed a second picture of their spouse as their brother. and when i go into CAS full edit mode to try and edit their relationship, it doesn’t change. this has happened to multiple relationships and it really sucks and while in this example their partner is a servo it has also happened which human partners

Hi!

I had my main sim adopt a child. When the adopted child came home, I had my sims’ partner (they’re both women if that helps) use the adopt interaction with the child as I assumed that would make them both parents of the child on the family tree. However, this made both of my sims in the relationship sisters for some reason. I have tried cas.fulleditmode to set their relationship back to household members which doesn’t work, and my main sims father is now the father of her partner as well on the family tree :’)

I don’t use any mods or CC, I play with them turned off and I’m on PC

I noticed my married sims couldn't do romance interactions, I checked and they are now labelled sisters as well as spouses/soulmates/etc in their profile. They are still spouses and unrelated in CAS but I can't figure out how to fix this. 

I see this issue has popped up for a few other Simmers but some methods I tried already didn't work.  I don't use CC nor mods.  I tried moving my family into another house and that didn't fix the problem.  I also moved my entire Sims 4 folder onto my desktop, started a new game so it would create a new folder, exited the game and deleted everything in the new Sims 4 fold and copied everything in the old folder (not the folder itself) into the new folder and repaired the game.  Then restarted it and only "Offer Massage" is the only Romantic option available between my Sims couple. 

What else is there to try?  I'm officially at a loss.

Have your sims become siblings in the family tree? Try checking to see if this has occurred.