Some of the answers were so quick to judge. We don't know why she wasn't married to the boyfriend. There is always reasons, but it is not the question here. The question is about the youngsters growing up around them. Since the niece didn't go on vocation with them, I assume they don't live together, so they grow up like cousins, maybe close cousins. From the sound of it, they love each other really a friendly way. I think the boyfriend is kind of irresponsible to even suggest it. Since you're not married and there is no blood related, I wouldn't worry too much what direction it takes. Don't even make a fuzz and don't intervene because it would be useless if something did happen. I kind of feel nothing would happen though.