I also have this issue, i do use mods, and i havent quite isolated what caused it yet, havent quite messed with it yet. all i know is that sometime after all my neighborhood teen sims grew up to young adults and my children grew up to teens, suddenly my (former) kids thought they were siblings with all the unrelated teens in their household. and the (former) teens could not do hardly any romantic interactions with their love interests without glitching the heck out. i found this thread and deleted their relationship and used cheats to redo it and it worked again. so im guessing they also decided they were suddenly siblings with their love interests >< EVERYONE decided they were related lol. i did go into cas after aging everyone as the main household i was playing had the 2 main children i was playing and after aging them up i wanted to redress them. so it is possible like with everyone else it was caused by CAS.
I know its not super helpful, just wanted to add my Me too anyways.
