Please use this thread going forward: Partnered Sims turn into siblings | EA Forums - 4943482
Unfortunately just got the same bug on my male/male couple. I did go to cas to give them new outfits, but I didn't notice the change immediately, because I was too focused on their baby. I only noticed when they went on vacation and there was no option to woohoo. I thought it was vacation rental lot bug and tried again once they got home. that was when I finally realized they had a sibling relationship. I cheated it away, but they still can't do anything romantic. I do use mods, so I can't be off much help, but yeah, this is apparently still happening.
Upon reading some other posts, I assume it must be the stepchild problem in my case. One partner was an adoptive father. After some time, I made the other partner an adoptive father too and everything was fine. Then I went into cas and that forced the silbing relationship. I mean, it¨s good that I know about it now, but apparently it cannot be changed even with cheats.
