13 year old sister is having with 26 year old bf?

I am 15 and my lil sis is 13. When I was coming home I heard moans and when i went in i saw my sister in a very advanced position. I feel really stupid because my mom expects me to literally always be there with my sis. She's only 13 I know she knows about but i neva expected her to do it! Is she old enough or should I tell my mom?