Advice needed: I think my sister is having with our new stepbrother?

I'm a 19 year old girl and since my 16 year old sister's room is next to mine I've been recently hearing noises from her room at night I can hear through the wall. I thought she was just masturbating, but when I walk by her room the next morning, I noticed our new stepbrother who just turned 14 this week is sleeping in bed with her and they are snuggled up together. He's wearing boxers and my sister wears a nightshirt to bed so they weren't naked or anything but it just seems really suspicious to me. Do you think my new stepbrother's curiosity & my sister's boy-crazy hormones are a bad combination and those two might be up to something, or could this all just be innocent?