Then she's a shitty mother.
EDIT: not to be rude or anything...
she used to be worse, much worse actually
Take your sisters phone and text him telling him to meet you somewhere and then bring a baseball bat and beat the shit out of him and get a bunch of paper clips bent in to the shapes of letters and brand "stay away from my sister" on his dick.
one time i found out that a 19 year old was hitting on my sister so i took her phone and called the dude, this is how it went...
Fag: "Hey babe"
Me: "Hey bitch, what the fuck does your sick ass think your doing hitting on a fucking 14 year old?"
Fag: "What? Who's this?"
Me: "Who i am doesn't matter bitch. What does matter is the fact that you're talking to a fucking 14 year old. If i find out that you EVER talk to her again I will come to your house, tie you up, slaughter all of your siblings and pets in front of you, curbstomp your mom, cut off your dads dick and shove it up your ass, and then... I'll fucking rape you"
Fag: "What no man, i don't want any trouble"
Me: "Good bitch, you know what to do" *click
he ignored all of my sisters calls for 3 months