Don't know what to do

My step daughter just turned 2, she has recently started doing something very unsettling... She'll be playing "peek a boo" but she'll say your name then when you look she'll spread her legs wide open and say "peek a boo!" Me and my husband want to confront her mom about this but with her age she'd just try to say she got it from our house when I can guarantee 100% it ain't from our house, it's just unsettling to me cause she's only 2, she use to make her dolls "make out" and will left her shirt and say "my nipple" my son is 4 who I have sole custody of doesn't do any of this so it shows it's definitely not from here.... All me and my husband do in front of the kids is kiss and hug