I wouldn't judge you if you did accept the blowjob from him, as a gay man I personally don't think it would have been gay of you to accept it. With that being said that's pretty fucked up with what he did to your sister, how to get back at him? Simple, snap a pic of him giving you a blowjob and blackmail him
Also you guys have the most stupid fucking gun laws ever, irrelevant but just saying