Bradbury (and Clarisse) never actually come out and tell us. It's not like Clarisse says "the relative that influenced me most was..." So we have to figure it out from stuff that she actually does say.
I think that it is probably her uncle that has the greatest influence on her. The reason I say this is that she talks about him a lot. She talks about him more than about her mother or father. Her uncle says that seventeen and crazy always go together. Her uncle says things were not always how they are now, etc.