The way I see it, you have quite a few options.
1, If he's anywhere near your size or strength, kick his fucking ass so hard that he coughs up shit and blood for life.
2, If he's larger than you, get him to do something violent to you, and call the police on his ass.
3, If you have access to Photoshop, and have the skills, make an awkward picture of him, and show it to your sister.
4, Assuming your sister isn't a bitch, tell her about this shit, and try to force a breakup.
And then there's always the option involving a pistol, duct tape, and a garbage bag.