Sarah Michelle Gellar has paid tribute to her onscreen sister Michelle Trachtenberg on her posthumous birthday. The Golden Globe nominee, who played Trachtenberg's sibling in Buffy the Vampire Slayer, recalled meeting her years before the famous show on what would have been Trachtenberg's 40th birthday on Saturday. "When you turned 16, it was hard not to still see you as the little girl I met on All My Children," started Gellar on Instagram. "When you turned 21, I had to teach myself to not see you as the little sister anymore. I can't imagine what 40 would have been like. This I know... it would be filled with love. Happy birthday @michelletrachtenberg." Trachtenberg died at 39 in February, when she was found unresponsive in her midtown Manhattan apartment as a result of complications from diabetes mellitus. A child actor since age three, Trachtenberg joined the cast of Buffy the Vampire Slayer with Season 5 in 2000, playing the Slayer's younger sister Dawn Summers. Following Trachtenberg's death, Gellar paid tribute with the words Buffy tells Dawn just before sacrificing her own life to protect her sister in the 2001 Season 5 finale, The Gift. "Michelle, listen to me. Listen. I love you. I will always love you," she wrote on Instagram.