I think it is different in this situation as you didn't grow up with your sister and you don't really know sisters bf as they didn't have much to do with your DS the whole time since his birth. Go with your gut instinct. Maybe just try and make a joke about it? It would also be different if your sister and bf had kids of their own but for him to want to take him out alone and then be rude when you say no is a bit weird. Probably perfectly innocent but who knows. I would feel weird asking if it was my in laws kids but I was asked to babysit while DH took his sister Xmas shopping (she is a single mum) after not really knowing the kids that long (they visited from UK).