National Sibling Day must be a major event in the Lively household. There are just so many of them.You might have known that Blake Lively 's older half-sister is actress Robyn Lively of Teen Witch fame . Then there's half-sister Lori Lively, whose acting credits include parts on Melrose Place and The Mentalist. Half-brother Jason Lively played Rusty Griswold in National Lampoon's European Vacation. Brother Eric Lively, meanwhile, has acted on The L Word, 24: Redemption, and Full House. Mom's family photo album must have a lot of headshots.With all that brotherly and sisterly love floating around, the former Gossip Girl star decided to share a family pic in honor of National Sibling Day yesterday. Judging by the caption, she's fully aware that she comes from a big family."Happy National Sibling Day," she wrote. "Yes, our parents are breeders."