if you tell him, you scare her off to the fair, giving you access to live there. if you become leader without telling bramblestar, she can have her kits in brambleclan but you cant get into the fair
Who does your sister have kits with? Sunfall. If you keep her in the clan, she will have the kits (Moonkit, Wisteriakit, then 2 others I can't remember the names of) but Sunfall won't come with her. If you tell bramblestar and send her to the fair, she will still have the same kits, just in the fair.