Kutcher, 35, joined Kunis, 30, in St. Petersburg, Fla., on Saturday to attend her brother s wedding.
Michael Kunis, 36, tied the knot with ballet dancer Alexandra Blacker, 29, who trained Mila to play a ballerina in 2010 s Black Swan, at the Cathedral Church of Saint Peter. The couple got engaged last December in San Diego.
Following the ceremony, to which Mila wore a blush, knee-length dress, a reception was held at the historic St. Petersburg Yacht Club.
According to a source, Mila and Ashton’s present to the newlyweds is their upcoming honeymoon: a 10-day trip to Australia.
“Ashton and Mila have talked about getting married for quite a while,” a business source close to Kutcher tells PEOPLE. Adds another Kutcher pal: “They’re incredibly happy together.”