Brielle Biermann celebrated her 21st birthday in style on Saturday ... I don’t know her!” the birthday girl — who recently announced she’s moving out of her mother Kim Zolciak-Biermann ...

"Happy 21st birthday to my beautiful Sami girl," Denise wrote in the caption of the post. "You were on the cover of People when you were born and here you are at 21 with a show about our family.

Lisa Rinna is celebrating younger daughter Amelia Gray Hamlin on the model and actress's 21st birthday ... "The soon to be Birthday Girl in soho today @ameliagray @alexanderwangny ️ ...