Advice needed about 17yo sister letting 13yo stepbro sleep in same bed?

I've been noticing lately that on most mornings when I walk past my sister's room after waking up, she will be in bed asleep with our new stepbrother. She only wears a nightshirt to bed without underwear and only he sleeps in boxers and one time I found birth control in her room and she doesn't have a boyfriend, so I'm wondering if this is just an innocent thing, or should I say something before curiousity & hormones get the better of those two...that is, if it hasn't happened already? Should I talk to my parents, or keep this just between us by only talking to my sis & stepbro?