Is it OK my 17yo sister lets our new 13yo stepbrother sleep with her?

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 brother. She only wears a nightshirt to bed without underwear and he only sleeps in boxers, 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 them...if it hasn't happened already? Should I talk to my parents, or keep this just between us by only talking to my sister & stepbro?