You sister is simply experiencing the thrill of orgasming, and cannot get enough, she does not need help. Education is what she needs, be honest with her and tell her the fact of life, then she will be better prepared for her sexual adventures.
if she is 14 nd masturbates a lot it okay, it a part of puberty ... but try to keep her from having sex until she is more mature.
It usually runs in the family. Do you have an older sister?
She sounds like a normal teenager, I hope she uses protection.