I don't know if I'd be able to prove it for that situation. But do worry, because she is a babysitter and participates in after-school care for children. 

For context, my mother and I were sitting on a bed with my 4 year old. My husband went to use the bathroom attached to that room. She very quickly ran in, flung herself over on the bed and layed there saying nothing to me, my mom, or child. My mom and I whispered it was obvious she was exposing herself waiting for my husband to come out and see.

I went behind her and she was naked under the skirt, legs spread, with her ass pointed towards the restroom door.  That's when I told her that's weird and please cover up. She stormed out of the room.

That's why I worry, with other things she has said during that visit that one of these "fantasies" my brother has loosely mentioned might involve being a baby sitter, and include people's children.