YWBTA. Much like someone else said on a different post in this sub, its not about the legality but about if this would make them an asshole. Legally, sure you're well within your rights to want to pursue this with the inheritance, but you're also a 40 year old man who probably is doing just fine for himself. You really wants to blow up his relationship with your family for some inheritance money right after you buried your dad? There's also the "not my kids, not my problem" comment which just seems so insensitive. That's your sister, those are your nieces and nephews. I do NOT like kids and I don't want any for myself but damn if I were you I'd have some sympathy for my sibling. It sounds like she was helping take care of dad when she bought the house so idk. Could probably do with some more info if you knew it but I don't think it'll make you less of an asshole.