"I am very excited to say, last week I asked my girlfriend Brenda Song to marry me and she said YES!" Cyrus wrote. "We are both very excited to be engaged!"
Miley was quick to publicly congratulate her brother, welcoming Song into the family. "Can't wait for Brenda to be a Cyrus... couldn't be more excited about my 'sister in law to be' yay," she tweeted.
Song, 23, and Cyrus, 22, made their first public appearance together in May at the Nylon party but they've reportedly been seeing each other for months. In August, a report claimed that the couple was expecting their first child.
Besides The Suite Life and its spin-off, Suite Life on Deck, Song has also appeared on Disney's Hannah Montana (starring her boyfriend's famous sis), The Wizards of Waverly Place and the TV movie Wendy Wu: Homecoming Warrior. On the big screen, she recently played the clingy, fire-loving girlfriend to Eduardo (Andrew Garfield) in The Social Network.