Should I be concerned?

A few weeks ago while watching my nieces (3 & 5) the older one said she wanted to have underwear time and proceeded to take off all of her clothes (down to her underwear) and run around my house half naked, even shaking her butt in my face at one point. Her little sister didn’t copy her which I found weird because the little one literally does everything that my older niece does. They have 2 different fathers, on the weekends he older one stays with her biological dad. So I figured this is a habit that she inherited at his house.

I talked to their mom about and explained how wrong that sounds and she thinks that’s what her dad says when it’s time for bedtime/bathtime.

So the next time I watched the girls, she did the underwear time thing again. Based on what her mom said I told her that we weren’t going to have underwear time right now because we are not taking a bath or going to bed.

Now today she pulled her hair up and said to me, when I have my hair like this I look 18. This just brought the whole underwear time issue back on my mind. Like who and why is someone telling a 5 year old that they look 18? Now I know kids say silly things so am I overreacting? Or am I right to be concerned? Her mom doesn’t seem to be concerned about it. (I do not have kids btw)

What are some signs of potential sexual abuse that I should look out for?