Selena Gomez does have two sisters, yes, and both are her step-sisters.
When Selena was five years old, her parents - Ricardo Gomez and Amanda 'Mandy' Cornett - got divorced, and her mum, Mandy, got custody of her. At the time, her parents didn't have any other children, so she spent most of her life growing up as an only child.Her mum later remarried and had another daughter with her new husband, Brian, and they called her Gracie Elliot, who was born last summer.
Her dad also remarried to a woman called Sara, and they also had a daughter, called Victoria, and she was born in June this year. So whilst Selena Gomez has spent a lot of her life as an only child, she's recently become an older sister to two little babies!
She's going to be releasing a Greatest Hits album at the end of the year, which should have two new songs, so maybe we can expect some of that new music to include her adjusting to sisterhood in her twenties!