How can I talk to my niece about the way she dresses when she babysits?

I have a 16 year old niece who has been babysitting my two sons for the past few months. My sons are age 5 and 7. A few days ago I was talking to my 5 year old about his cousin and how he liked being babysat by her and out of the blue he told me that he saw her . That was pretty shocking so I asked him how he saw it, thinking that maybe he had walked in on her in the bathroom or something. He told me that he had seen up her skirt and that she wasn't wearing underwear. I thought that he might have just been making it up so I asked my older son he told me the same thing. At first I was worried that something might be wrong and that maybe she was showing them her privates on purpose. I asked them for more details and from the way they explained it it sounds like they saw up her dress by accident while they were playing. Even though I was concerned about why my niece would be wearing a dress with no underwear while she babysits, I wasn't really sure whether to confront her about it or not thinking that maybe it was just a one time lapse in judgment. I know that she likes to wear short shorts and skirts so maybe she just forgot to put on underwear that day. I did start to pay closer attention when she came over though. Then today when I came home I noticed they were wrestling around in the living room. My niece was wearing a skirt and rolling around on the floor. At one point I noticed her skirt ride up and I could see that she definitely wasn't wearing anything underneath. I'm sure my sons noticed as well so right away I came in and told them that play time was over and I told my niece she should go home. I was so flustered that I couldn't confront her about it at the time. Now I'm trying to decide whether I should say something about it or whether I should just tell her I found another babysitter. How should I confront her about it, or should I tell her mom? I don't want her babysitting my sons dressed like that. She either needs to put on underwear or stop wearing skirts when she comes over, but I'm not sure how to tell her that without being awkward.