i feel like he has had to do a lot of assuring people that he is nothing like his sister ... at least where bullying people is concerned. so he'd probably be happy she has no grudges. but yes! i could definitely see them knowing each other from being kids. he was pretty quiet and reserved as a kid since he felt really out of place in his family, and then he came into his own at hogwarts when he got out from under their influence and into ravenclaw. and i think he definitely appreciates any fellow pureblood who isn't a total bigot at this point. he's like oh, you get me AND you're not insane. thank god.