İf İ were you, here's what İ would do:
1. Kick that guy in the balls And throw him our the window
2. Have a serious talk with my Sister telling her that shes too Young, its illegal, she could get an std, she could get pregnant , And if you're christian or Muslim, tell her that its a big sin to have sex Before marrige.. İf she really HAS to have sex she can masturbate.. At LEAST she can use a condom.
3. Tell My mom. ( careful not to tell ur dad... He might freak out more than ur mom)
4. Hope For the best
5, ur sis will thank U For this someday...
Good luck bro