"I'M GONNA BE AN AUNT," Camille Schenkkan tweeted

Credit: Gilbert Carrasquillo/FilmMagic

Gotham fans aren’t the only ones freaking out over costars Ben McKenzie and Morena Baccarin‘s baby on the way.

The O.C. alum’s sister, Camille Schenkkan, tweeted her congratulations to the happy couple – and the dad-to-be retweeted her sweet message!

“Couldn’t be happier for @ben_mckenzie and @missmorenab. Welcome to the family, Morena! Also… I’M GONNA BE AN AUNT,” Schenkkan wrote Thursday.

McKenzie, 37, and Baccarin, 36, kept their relationship under wraps until last weekend, when they were spotted getting cozy at Emmy events. On Thursday, PEOPLE confirmed the actors are expecting their first child together.

Set photos of McKenzie and Baccarin – showing a sizable baby bump beneath her coat – filming Gotham last week have also emerged.

“She and Ben are very happy about this news,” a source told PEOPLE of Baccarin, also known for her work on Homeland. “Everyone is very excited.”

Baccarin already has a son, Julius, 23 months, with her estranged husband Austin Chick.