Partnered Sims turn into siblings

This has happened to me too, but on PC! No matter what I do it always becomes an issue all over again. My sim's wife will become her sister and then they can no longer have romance interactions. I've seen mixed reports on the issue, ranging from those who think it's base game related to mod related... Personally, it's been a problem for me ever since I accidentally made them sisters through MCCC by clicking the wrong relationship bit. I've tried fixing it through cheating the relationship bits, deleting the sim and adding her back from the gallery, clearing her relationships and starting over on everything, and even trying to use cas.fulleditmode. None of this works, it always reverts again. It's on my one and only legacy save and I've read that others have the issue come back either when they go into CAS or after their sim sleeps through the night. :( This is a really bad game breaking bug, and removing MCCC doesn't help fix it either. 

did you manage to resolve this issue because i've been doing a generation game and my matriarchs (with no prior relationships) keep refusing to woohoo because they have the sister trait despite them not being sisters. 

After my scientist sim made a clone of herself, her and her wife are listed as sisters and can no longer have romantic interactions in the game. Only way to fix was to download a previous instance of them that I saved, even downloading MCC and choosing their relationships did not work. The family tree showed a duplicate of my sim and showed one as being married to my other sim and one as the sister.

I had turned off my PlayStation and/or put it on rest mode and then next morning I tried to play sims my sims spouse had turned into his brother. I don’t want this to happen I wanted them to be spouses not brothers what’s going on with this?

Edit By Crinrict: Adjusted Title fit bug report at AHQ

this is still happening for me despite the most reccent patch claiming it has been solved

I have this issue too. The sim I'm playing got a romance bar with his sister and with his father. And when you look at the relationship tree they're gone. 

Yeah, I have a young adult/teen relationship that's continued after age up as well as related ones I've seen happen in public spaces with played households too... Autonomous inappropriate romantic interactions. I decided to shut er down and wait for some bug fix updates as with all the other glitches I fear breaking this save.