I’ll give you another perspective I’m 26F so maybe I have a little more of a perspective . My MIL has some of my husbands flings on her FB she’s just one of those women who are super friendly . At first it bothered me but she still post about me and her son a lot and I don’t think she’s doing it in a malicious way . Most of the time the exes end up taking themself off . It would be different if this ex kept trying to intervene but at that point I’d leave it up to your fiancé to ask his family to remove them off social media . Chances are his family would respect his wishes . If not then I think there might be more pressing issues to unpack dealing with his family .
Your not being ridiculous! Your pregnant with his baby and when you have your LO your in laws will probably be wanting to post pictures up on their socials of your LO and the ex will probably try to do something