Okay so I've never had this problem before and I've played Sims 2 since it came out, I've got Sims 2 Ultimate Collection so I don't have to use all my brothers expansions any more, I've started up a neighbourhood and married two sims, and now the male can't perform romantic interactions on his sister-in-law, but can perform Family kiss, despite the fact that this shouldn't be happening and she's not listed under family in his relationships tab. She can still perform romantic interactions on him. It works exactly the same the other way round in that the wife can't perform romantic interactions on her new in-laws either but they can on her.

If anyones got any ideas on whats causing this or how to fix it that would be appreciated, thanks.