Selena's close friend, Demi Lovato, appeared to let slip the exciting news after congratulating Selena, Mandy and her husband Brian on Twitter late last night (13 June).
Pop star Selena has not mentioned her new baby sister on Twitter, although she was apparently spotted visiting Mandy and her sibling in hospital.
Last month, Selena revealed that Mandy was pregnant by sharing a picture of them cradling her bump to mark Mother's Day in America.
RT #CONGRATSSELENAMANDYANDBRIAN !!!!!!— demetria lovato (@ddlovato) June 14, 2013
The little one's arrival will bring much joy to Mandy and her family, especially as she tragically miscarried a daughter in December 2011.
The couple named their baby Scarlett and Selena cancelled her workload to support her loved-ones following their loss.