Sexual 7 year old?! Please help!

Hannah • Married to a wonderful, hard-working man, with a rainbow “ours” baby boy 07/11/19 🥰 and two beautiful step daughters. 💜

I recently caught my step daughter playing with herself on my couch. Didn't think much of it. I just explained to her that it's something you do in private.

Well, caught her today playing with her 3 year old sister. I don't know how to approach this. Can someone please help? I don't know where she learned this, or when. We only have her every other weekend.