I'm sure I would hate my sister if she wasn't my sister
I really hope that's a joke. What about your sister makes you think you would hate her?
I am just utterly disgusted by her boyfriend, even though I've only met him once.
If your only concern with your sister is that you hate her boyfriend, then you shouldn't go so far as to say you hate her. It is bad to hate your sister. Try not to judge her too harshly for her boyfriend; he might not actually be that bad, and she might not be with him forever anyway.
I don't want her to keep dating him
I get that you don't like her boyfriend, but even if she stays with him it's no reason to dislike her. You should still love your sister. Tell her you'd like to spend time with her without him present.