She is but she's the result of constant emotional abuse. Grandpa is the real B.I.T.C.H. Just from this brief interaction we get a good look into the aunt and moms upbringing and how poorly he treated his kids.
Seems clear that mom left to escape grandpa's abuse just to marry into another abusive relationship. From the phone call and dinner dialogue it seems likely everyone knew mom was getting abused but didn't care. Perhaps even thought she deserved it for trying to escape. Abuse victims are prone to getting entangled with abusers and perpetuated that abuse themselves onto others. Aunt is a bad person but I feel is sympathetic and likely resented her sister for leaving her alone in that garbage household with grandpa. It's hard to be a good person when you're regularly being abused. Just look at the mom overwhelmed at her own situation and lashing out onto her child, the only one who even cared about her.
Yeah I get you, the grandpa is the real villain
Damn the aunt is such a bitch