Have there been any hints in the books up til now though? I mean, I don't even recall Neville mentioning a Harfang Longbottom at all. If he had then that might be a dead give away. He tells us about one other relative - sorry too lazy to go look up the name - besides his grandma -and we don't know what side that relative is on. As for the Longbottoms being related to the Blacks... doesn't Sirius say in book 5 that all of the wizarding families are related? If pure bloods are only going to marry other pure bloods then they don't have many options really.