I caught my 13 year old sister having sex?

My parents went on a business trip out of state for three days and left me in charge to look after my 13 year old sister. I'm a 17 year old boy btw. Well yesterday when I got home from I friend's house I heard my sister yelp really loudly from upstairs. I thought she hurt herself, so I ran upstairs and saw... show more My parents went on a business trip out of state for three days and left me in charge to look after my 13 year old sister. I'm a 17 year old boy btw. Well yesterday when I got home from I friend's house I heard my sister yelp really loudly from upstairs. I thought she hurt herself, so I ran upstairs and saw her in her naked in bed with some boy on top of her. So I was like WTF and threw the boy out of the house and my sister begged me not to tell our parents... I want to give her a break but I don't want her to be seeing this kid anymore. What should I do? :\
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