We know that even in SS, people who can use their rieatsu are special. We've seen this in several flashbacks. We've also seen the "normal" people in SS, who are just that. Normal.besides, i dont think souls just magically gain huge amounts of rietsu when they get to soul society
Also, the Kurosaki's obviously aren't the 4th family, as Byakuya would have known it and not bitched about Ichigo being so naturally strong. I'm thinking it's either the Genryusai (Yama-jii) or someone unimportant.