Taylor Swift's Sweet Birthday Message to Selena Gomez | Selena Gomez, Taylor Swift

Taylor Swift and Selena Gomez

Courtesy of Taylor Swift

Taylor Swift's Sweet Birthday Message to Selena Gomez

Tuesday July 23, 2013 08:00 PM EDT

They don't look anything like sisters, but Taylor Swift says they could be.

The singer Tweeted a cute picture of herself and Selena Gomez on Monday, with both stars perched upon a balcony above the sea while striking a pose to imitate the mermaid sculpture between them.

"Happy 21st birthday to my friend, and the closest thing I've ever had to a sister, @selenagomez!!" Swift Tweeted.

The pair have been friends for roughly five years, and seem to be very supportive of one another. They often say sweet things about each other:

"I met [Swift] when she was 18. This is before [Swift's 2008 album] Fearless came out, and it was absolutely incredible to see someone so successful and so humble," she says. "I think she has been a big part of me kind of staying the way I am too," Gomez has said in the past.