She wrote: "Momma, Brian, and me are all so grateful for all your well wishes. My lil sissy Gracie Elliot Teefey was born on June 12th. Luv u all XO."
Mandy divorced Selena's father, Ricardo Gomez, when the Love u Like A Love Song singer was five-years-old, before marrying Brian Teefey in 2006.
Demi Lovato was quick to "congratulate" her life-long friend on becoming a "big sister" when she made the commentaar on her own Twitter account today.
She tweeted: "@selenagomez IS A BIG SISTER!!!!!! Everyone congratulate her and her family!!!!!! #CONGRATSSELENA!!!!"
Speaking before Gracie's birth, Selena admitted she would "spoil the baby" and wanted to be "more like a cool aunt" in the future.
She previously said: "I want to be the best sister in the world. I'm really going to spoil the baby! I can't wait ... There's going to be a big age difference, so I think I'm going to be meer like a cool aunt
