I can’t stand his sister?

I hate my boyfriends sister with a passion and it scares me because I don’t want to get married into a family that i don’t like. He says he hates her too but she’s so annoying. She sits home lives there with the grandparents and doesn’t work neither does her ‘boyfriend’ and they have the family supporting them. Now they tried for a kid and she’s pregnant and she doesn’t see what she’s doing wrong by bringing a kid in the world when she can’t support it. Her excuse it she’ll live off the government if anything. I know its not my business but when you really hate someone so much everything they do IRRITATES you. Any time I mention it to my boyfriend he gets annoyed and says he doesn’t care and thinks they should work and all too but never cares. I never go in his house anymore to avoid her. what would you do? she’s very obnoxious and tries to purposely get me upset by stuff she says. Like me and my boyfriend are trying to get a place and her comment was oh they’re not married. I’m so sick of her and the nonsense advice please...