Is it Halaal to get revenge right? Like this guy keeps saying stuff to my sister and I want to get him back because my sister won't get him so I can get him.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

First of all, we advise you to take the initiative to teach your sister the matters relating to her religion, because if a woman is steadfast on the obedience of Allaah and wears Hijab, then that will be a protection for her from dissolute people. Allaah says (interpretation of meaning){O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies. That will be better that they should be known (as free respectable women) so as not to be annoyed).}[33:59]; meaning in order that the dissolute people will not harm them. You can repel the harm of this person by informing the authorities who would prevent him from doing so. But you should not hit or beat him as this will involve you in being questioned or cause you other embarrassments.

Allaah knows best.