Help!!!!

My niece is going to be 3 next month. Her parents have been separated and recently finalized the divorce... Her mom is with another woman and has been for a while and her dad is single. They get the kids every other week (has a brother thats almost 5) anyways when they are with their dad we watch them while he’s at work so the only people that watch them is my parents (their Grandparents) their dad (my brother) and I. When they are with their mom I know the girlfriend (that we know thing but her name) watches them occasionally her mom and we’ll pretty much who ever else she can find to watch them. (They run a muck and do as they please over there) During the school year we watch them on her week til 3 or so but now that it’s summer she’s On her own as she lives 30 mins away (they goto school in our town as my brother has residential custody). Any ways now that there’s a little history.... my 4 year old who is very private about her body told me the other day that her cousin (the girl) opened up her hooha and was sticking her fingers inside her hole! Major red flag to me... I talk to my parents and they said yes my brother has caught her doing that to herself and has talked to her... now I know just like little boys they are curious about their body and are going to touch but what gave me the red flag was that she was sticking her fingers inside... my parents talked to my brother as he’s suppose to talk to her mom ... I told them he should take her to the doctors for a check up.... I talked to my niece the best to her understandings that she’s not suppose to do that and no one else is suppose to either and she vered away and was very shy about it and she’s not a shy little one! My daughter piped in and told her only the doctors or her parents can look down there only if it’s bothering her.

Am I over reacting or should we be concerned about this... I can’t let it out of my mind.

**update! Not sure how to respond anonymously... my parents have talked with my brother and he’s suppose to have a sit down with their mom and go from there... also I too never knew there was a “hole” down there til probably 5th grade lol always thought that’s where my lee came from haha thank you for the responses! **