For 15 year olds?

I have a 15 year old sister that lives with my mom and step dad. she had a hickey on her neck the other day from her boyfriend who my mom lets come over to the house when its just my sister and him. i really want to trust her not to be doing the things she shouldnt be, but with kids these days i dont really know. i wouldnt have even thought about it when i was that age and i'm 21 now. my dad wants me to take her to get on birth control but i feel like that will just be giving her permission to go have and not worry about getting pregnant. on the other hand if she is then she's bound to get pregnant anyway. i dont know what to do