Leon has no actual siblings, but he does have a baby cousin who essentially fills the role of "sister"!

Too bad Kanon doesn't see him that way, heeeeeh...

Noodle has no blood family that we know of, but she does explicitly refer to her bandmates as her brothers! 2D, especially, fills this role.

(According to her, she's the big sister to them - comforting 2D about bad dreams, Murdoc about bad trips, and Russel whenever he's feeling down about the state of the world.)

Santa's little sister is his partner in crime and one of the few people he really cares about. He's at the top of his game when being a good brother, as anyone who's played the game might know. It's really sweet.

And Trent is very close to his sister Jane! They have several older siblings as well, but those are all grownups, not around very much, and each have their own issues. The Lane family is kinda screwed up.