I don't even think I could talk to him if she tries to bring him over.
DON'T. Seriously. Don't. If you're the nice one and even you don't make an effort with this guy - again, another little push to get her to see the light. If you're nice to him so that he feels "welcome" - TRUST - she will see it as "Oh... sis really does kind of like him, despite what she says.". She will grasp on to EVERY little sign that he's accepted.
I honestly don't think I could even be polite to him. She *knows* I loathe him. My plan is to just ignore him if he does come over. I won't be mean, but that doesn't mean I have to be nice either.
Don't play these games. There is absolutely positively no reason you should be inserting yourself into your sister's relationship. Is she going to ask you to call him when she wants to break up with him too? My BFF did that for me once. The difference was that I was in elementary school.
This is also reminding me of a time (recently) when I was accused of "sending someone straight to voicemail." Ummmm... There are a million reasons why you would be sent to VM. My phone could be off. I could be in the subway. I could be in an area with no service, which is rare, but does happen when I travel or go hiking. I could have my phone on silent. I could have left it at home. But, of course, you are going to jump to the conclusion that I'm ignoring you. And guess what? If I have no record of a missed call (which is the case in many scenarios where you would be sent to VM) and you don't leave a message, I DON'T KNOW YOU CALLED. So, yeah, I'm not calling back.