Advice needed about my sister letting stepbrother sleep in bed with her?

I'm a 19 year old girl and on mornings I wake up before everyone else in my family, when I walk by my 17 year old sister's bedroom and look through the door open a crack, I usually see our new 13 year old stepbrother snuggled up to her sleeping in the same bed. My sis is wearing her nightshirt and my stepbro wears his boxers to bed, so they weren't naked or anything, but do you think there's any chance they could be up to no good? I think my bro's young curiosity and my sister's hormones making her boy crazy recently could be a bad combination, and it could lead to an "oops accident" and my sister might end up pregnant. Should I talk to my stepbro and sis privately to keep it just between us, or talk to my parents?