Honesty all the way. List your reasons to her. It's you, not me. You've let yourself go. I fancy your sister as well. When we walk down the street there are so many fitter burds out there. Your moustache tickles and catches on my pubes. I prefer spending time with my PS3, my mates, myself etc. No offence but I could do so much better. I hate it when you cry.
You need to eat one first. [video=youtube;7yl3UMO-TkE]http://www.youtube.com/watch?v=7yl3UMO-TkE&feature=related[/video]
I've done that more than once. It's the real ****bag's way out, takes ages and leaves a lot of ill-feeling. On the other hand, it's pretty easy to ignore them shouting through the letterbox while you're hiding behind the couch. Why not suggest swinging? Best case scenario is she dumps you. Worst case is you get to effectively pimp her out in exchange for a go on someone else's. And then you can dump her for being a whore.