Actress Tia Mowry got engaged to her boyfriend of six years, actor Cory Hardrict, on Christmas day, she tells PEOPLE.

"He surprised me in front of my entire family," says Mowry, 28, who starred with twin sister Tamera on the 1994-1999 WB series Sister, Sister and is now on the CW comedy The Game.

"I knew something was up because he came over to my parents' house and he was acting strange. He went to the bathroom three times in 10 minutes and he was sweating profusely. I was like 'Cory, are you okay?' "

Turns out, acting funny was Hardrict's master plan. "He's a jokester," says Mowry, who costarred with Hardrict, 27, in the independent film Hollywood Horror. "He had given me a card and it read, 'I love you, blah blah blah.' And he said, 'Tia, I just want you to know that I want to be committed to you, and with this promise ring I want to let you know that I love you and I want to work towards the near future.' "

Only instead of a ring, Mowry's little brother came over and handed her a piece of paper that said "Psych!" while Mowry's other brother queued up the Jagged Edge song "Meet Me at the Altar in Your White Dress."

Hardrict then got down on one knee and gave Mowry a diamond ring he'd designed himself. "He was crying, I was crying," says Mowry, who hasn't set a wedding date yet. "It was just one of the best moments of my entire life."