Chekov. Wesley would be HORRIBLE to have as a sibling ("Why can't you be more like your brother?"); Bashir, I lust after, so I don't want to be related to him; Kim is kind of a dufus, and I'd always be getting him out of trouble, and Reed is really uptight. Chekov would probably just be the most normal guy.
Would you rather have Uhura, Troi, Kira, Janeway or Hoshi as a sister?