"You may be taller then me now, but you will always be my little sister," Sarah Michelle Gellar says of Michelle Trachtenberg

By Michele Corriston
Updated October 12, 2015 11:15 AM
Credit: AKM-GSI

What’s “The Key” to the perfect 30th birthday? A heartfelt message from Sarah Michelle Gellar!

The Buffy the Vampire Slayer star celebrated on-screen sister Michelle Trachtenberg‘s big milestone with a sweet Instagram post on Sunday.

“How is it possible that you are #30 @michelletrachtenberg I met you as a little girl on #allmychildren and have continued to watch you grow. You may be taller then me now, but you will always be my little sister,” Gellar, 38, captioned her collage.

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Trachtenberg joined the supernatural cult favorite in 2000 as a magical object known as “The Key” but disguised as Buffy’s suddenly present sibling Dawn.

Gellar has been all about Buffy nostalgia lately, sharing a photo of a cake with figurines representing her character and David Boreanaz‘s dreamy vampire with a soul Angel in honor of National Angel Food Cake Day on Sunday. (“Oh wait? Is that not wait it means?” she joked.)

And last week, she reunited with Seth Green, who played rocker werewolf Oz, at New York Comic Con.