My sister is 12, she likes sex, i told her its not her right age, but she doesn't listen, has sex 2-3 times a week, also has a double penetration, is it harm?

Dangerous: It is very dangerous. She will be at risk to have sexual transmitted disease- like gonorrhea, syphillis, aids, chlamydia etc, she will be at risk to get pregnant of course. She is destroying herself-and she does not know even know that she is harming herself and other as well. Talk to her- if she does not listen- talk to your parents. Whoever is the partner- can be charged as she is under age.

Too young: She is a minor and can not give consensual sex hence having sex at that age with an adult should considered statutory rape. She should be refered to psychologist.