Why is my aunt like this?

She is always very disrespectful to my loving grandmother who I care deeply for. She is just downright cold and I hate to say this but completely bitchy. My grandmother does so much for her, she lives in houston and travels to austin just about every week to watch her kids. She helps clean around her house and even buys her stuff, my aunt says thank you but she still has this horrible attitude against my grandmother. Of course I would never say anything to my aunt for she is older than me and I must respect her and I would never intervene for that would make matters worse. But it just saddens me to see and hear what she says to her, so when I am around this I just ignore it or go into the other room. I don't understand..my whole life my grandmother has been a total hard (but in a good way), she knew how to discipline and she didn't take crap from her. Has she gone soft? My aunt hasn't always been this way either..she and my grandmother actually really got along. BTW my aunt has two children one is 2 and the other is 4 going on 5 and my grandmother likes and wants to be very involved in there lives. Is she afraid that if she says something to my aunt that my aunt will never want to see her and let her kids see her? I am not the only one who gets upset by this my mom gets extremely upset and is thinking about saying something to my aunt. Because of the way my aunt brings a negative vibe into the room and I've almost gotten to the point where I don't want to be around her..Is that right? that I don't want to be around my own aunt? I just don't know what her problem is..