Is it normal if i don't want to be friends with certain people?

So I have these roommates and Ive been having a blast with them. Honestly they're like good girlfriends that Ive never had. We go out together, we understand each other's sense of humor, we have that bond where we can sense each other if somethings wrong and we just look at each other, etc. However there's this problem that annoys me about them and is kind of stopping me from becoming even better friends with them. There is this other roommate that we can't stand, she's okay but she just gets annoying with her voice and attitude about things. I, as always, end up being the one to show I'm annoyed while the other ones act completely fine in front of her but when we're hanging out we all talk about her. So it just bugs me that they can't stand up for themselves. If you're gonna b*tch about it later, then it bugs you which is why you have to stand up for yourself. I want my friends to be able to do that. I understand if it doesn't bug you a lot so you don't feel like standing up for yourself but if you keep b*tching about it then you obviously care. So I don't know if I can be friends with them for long if they don't eventually stand up to her especially if I look like the bad guy when we all feel the same way. I want strong friends so I feel like I need to tell them even though I kind of already mentioned it but in a joking way.