Selena Gomez set to become a big sister

Singer Selena Gomez has a very special reason to give thanks this Thanksgiving - she's about to become a big sister.

Justin Bieber's girlfriend took to Twitter.com on Friday (25Nov11) to reveal her mom Mandy Teefey is pregnant.

She wrote, "Momma and (stepfather) Brian are finally letting me share the news. My mommys carrying my baby brother or sister. I’m the happiest girl in the world."

Gomez's tweet was accompanied by a black and white shot of her stepfather cradling her mom's belly.

Brian Teefey is mom Mandy's second husband; the 19-year-old's mom divorced her father, Ricardo, when she was five.