Partnered Sims turn into siblings

This had happened to me as well in the update! My sim Nakoki had (now deceased) parents and for some reason my other sim Johnnie who is Nakoki’s husband is seen as a brother to his husband and a child to the parents and they are supposed to be seen as a son in law to the spouse’s parents. Not only that my sim Nakoki is also divorced to his ex wife Miwa who had only 3 children together before they divorced and for some reason she is not in the family tree as divorced in that sim’s pov. The ex also used to married to another guy and they had twins before they were divorced as well and when I checked the family tree she was in it but in her family tree she does not recognize her first ex Nakoki or her other kids as family same with the half siblings. I never in my years of playing the sims had ever experienced this type of bug before! And I heard people dealt this many months before this update came out. Edit: I found out it is because they are the same sex which is why the game believes they are brothers but that what makes the situation even more ridiculous because how come it didn’t affect the other gay couple from cats and dogs? Genuine question…

Happened to me as well! Scary as it's identical to the old (old but currently unfixed) Get to Work bug where cloned sims become siblings to their spouse as well. Save File Ruiner because it's unfixable even with MCCC or uploading to gallery. 

My guess? An issue with "sibling-in-laws" incorrectly being tagged as "siblings" in the game. If your spouse's sibling is your sibling, then your spouse gets tagged as also being a sibling. 

See: Frida + Ulrike are CAS siblings with no parents. In addition to Ulrike's erroneous tagging, Frida should also be tagged as "sister-in-law", but she's tagged as "sister" instead. This is my guess as to the origin of the issue.