The best way for a child performer to break into entertainment these days, it seems, is to have a sibling who’s already made it — and sometimes that sib is superior. Kieran Culkin, brother of Macaulay and the rest of that sprawling acting clan, is getting terrif notices for his performance in the just-released “Igby Goes Down,” UA’s darkly comic update of “The Catcher in the Rye” setin the plummier precincts of Manhattan and Georgetown.And singer Aaron Carter has, arguably, surpassed brother Nick of the Backstreet Boys in popularity with the bubblegum pop crowd. Over at Nickelodeon, another relative of teen-pop royalty is making her TV debut on “All That,” the Tollin-Robbins produced sketch comedy series that spawned junior-set giants Amanda Bynes, Nick Cannon, Kenan Thompson and Kel Mitchell. On the Sept. 21 season premiere, “All That” adds Jamie Lynn Spears, the 11-year-old sister of Britney. Among other recurring roles, she’ll play “Giggles, the mischievous elf.” Can superstardom be far behind?