HELP!! I caught my 11 year old sister having sex and idk what to do?

So yesterday i finally got early dismissal for school so i came home early today and i caught her with a sophomore having sex on my parents bed. she immediately started crying and begged me not to tell our parents and swore she would never do it again, that she was just curious and the guy convinced her to. I am debating whether to tell my parents or not because my dad is supperrr strict and overprotective and will probably beat her or something! she didnt use protection though because i guess the gy talked her out of it and she trusted him. help me please i am really confused.