There's on thing in your letter that is an outright lie. You write that nobody in your family likes this guy. That's not true. Clearly your sister does. In fact, she must like him a great deal to stay with him knowing how much the rest of you hate him. This simple oversight reveals the entire cornerstone of your problem here.
You don't need advice on how to break them up; you need advice on how to shut the fuck up and pull your nose out of her business. You don't like him? Your mom doesn't? Your dad doesn't? Your uncles, cousins, and fucking pets don't like him? TOO BAD! Your sister does and you obviously haven't even taken her feelings into account.
Let's say I give you a brilliant plan to break them up and you go through with it and it works brilliantly as all my brilliant plans do. Imagine you've done this already. Fuck yeah, right?! Oh, but what's this? Now your sister's crying. She's heartbroken, upset, and alone now. HE didn't do that. The asshole boyfriend didn't break her heart. YOU, the wonderful loving caring brother, did that.
If she's happy, it's none of your business. Even if she's not happy, it's still none of your damn business unless he's abusing her or something. You end this relationship, she'll be in another one with another asshole because if she's not happy she needs to figure out FOR HERSELF that she's not happy and why she's not happy in order to avoid making the same mistakes again. These aren't things that brotherly love can force on her, it's something she has to learn by experience.
You and the rest of your family need to get the fuck over it. Who your sister dates, fucks, or even marries is none of your business. She's well beyond the age when it's neccesary for the whole family to look out for her well-being and is now at an age where doing so would only hurt her and, by proxy, hurt you.
In summation: Stay out of your sister's relationship. You're not dating her, so shut the fuck up.
All My Hate,