My little sister is having sex...should i tell my parents?

You should tell your parents, by showing the evidence, but to tell them not to rat you out. I'm the youngest in the family and I always told on my older brother when he did something that was not right for safety reasons. He was mad at sometimes, but as the years went by he isn't mad at me anymore and is very kind to me now. It's a good thing to be concerned about your sister, it shows that you care.