I've been having this problem too even with no previous relationship. The son (the heir) got married , new wife moved in and the father was constantly flirting with the daughter-in-law and she reciprocated. Wrecked the family. I would like to see an in law family dynamic added. That way if you want the drama, you can choose to ignore the in-law dynamic. Personally, I would prefer the in law respect
Yes, a simple add-on of a "chosen family mark" would fix a lot and could be used in so may ways (bff, adopted sibling, in-laws, chosen parent-child dynamics between adults and elders, etc..) ! The Sims has improved a lot as a safe space ans showing diversity these years, this really simple issue seems to be an oversight.
