Is it OK for my 17yo sister to let our 13yo little brother sleep in her bed 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 younger brother. She usually wears a nightshirt to bed without underwear and he usually sleeps wearing only boxers, I found birth control pills 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...if it hasn't happened already? Should I talk to my parents, or keep this just between us by only talking to my sis & our little bro?