Should I be concerned?

I have recently been sitting my nieces more frequently. They have two different fathers and the older of the two stays with her dad on the weekends. So a few weeks ago we were hanging out and she said she wants to have underwear time. She then took off her clothes and ran around my house in her underwear, one time even shaking her butt in my face. I thought it was weird because her little sister didn’t follow suit. They are 5 and 3 so anything the older one does the younger on follows. Well I talked to my sister about it and how it sounded and she explained that she thinks that’s what her dad says for bathtime/bedtime.. but she would talk to him about it. The next time she was over she wanted to have underwear time again. So going off of what my sister said I told her not right now because we are not taking a bath or going to bed.

Now today I am watching the girls again. The oldest was using the toilet when she pulled her hair up and said when I have my hair up I look 18. Now I know kids say silly things but it just has my mind back on the whole underwear time thing. Like who and why is someone telling her she looks 18? Am I over reacting? (I don’t have any kids of my own btw) or should I be concerned? What else should I be looking for in terms of sexual abuse?