should I tell my sister?

My nieces usually can open up to me about stuff without me running to their mom but I’m about fed up. I was usually here for them so they had an adult to talk to, since they felt they couldn’t share specific things with their mom.

But I just walked into the bathroom (it’s 5AM) my 13 year old niece is sleeping over and she’s in there on the bathroom floor with her pants pulled down taking nudes. She said she wasn’t but obviously is.

She has a 15 year old bf I bet they are for him. She just came here from his house last night.