My aunt is extremely rude?

Okay, so I have an aunt. And she lives with me and my mom. She was fun to have around at first, but now she acts like a teenage wannabe. She insults my mom and I, complains about everything, acts all snooty, she thinks she's so classy just because she's clean. I can hear her right now, talking about me and my mom on the phone and how our house is so old and messy. Like we're just normal, it's not like we're excessively messy or anything. I just wanna shout at her like what the heck is her problem? She came crying to my mom and my mom took her into our house. Now she's being a mean girl. -.- And I'm sick of it. My mom is too soft on her, saying that we need to be nice to her and she's a guest. My mom doesn't like her around either but she's her sister. What should I do? I'm sick and tired of this crap. She's annoying, rude, and a backstabber. I love her but it's hard to say so I hate her way of talking, help?