Sarah Michelle Gellar shared an emotional birthday message to her late Buffy the Vampire Slayer sister, Michelle Trachtenberg. "When you turned 16, it was hard not to still see you as the little girl I met on All My Children. When you turned 21, I had to teach myself to not see you as the little sister anymore," Gellar, 48, wrote via Instagram on Saturday, October 11, alongside throwback Buffy footage of the two actresses. "I can't imagine what 40 would have been like." She continued, "This I know... it would be filled with love. Happy birthday @michelletrachtenberg.