AITAH for going to my estranged grandmother's birthday dinner?

So, I (27f) have been estranged from half of my family since 2018. I grew up with a big Italian family. Lots of aunts, uncles, and cousins. Family parties almost every weekend. Coffee and biscotti and dessert at the end of the night. Keeping track in a little notebook of how much money a second cousin gave so I can look back and give them that same amount the next time they have a party. Mentioning that I'm Italian is unfortunately very relevant since we tend to be petty, transactional, and ALL about family (supposedly).

So my mom (56f) moved here from Italy when she married my dad (60m). And she claims that since then, his side of the family has mistreated her. I've heard stories growing up of them blaming her for silly things like eating the last of the salami in the fridge or "stealing" someone's clothes only to find out it was just misplaced (like I said: petty). I know there's been plenty more, I just have a hard time remembering all of them since I was either not born or too little/busy playing with my cousins to remember. My grandmother (dad's stepmom) and my dad's sister were the main villains in my mom's story. Over the years, my aunt has called my mom awful names, said she only married my dad for a green card, insulted my mom's family and so much more. My mom has been the PUNCHING bag of my family for a long time. Any time there's a family issue, she was seen as the cause even if it didn't seem to make sense. If she hung out with a couple of my aunts without the others, she'd get ATTACKED for being unfair. Even though those same aunts always got together without her and there was never an issue with that.

Now when it comes to ME....I looooved my family growing up. I felt rather alone and neglected at home so seeing my extended family was the best. I have so many fond memories of Christmas Eve at my nonno's, eating the 7 fishes and waiting until midnight to open the gifts, chanting for Santa with my cousins. We'd go on family picnics, spend every holiday together, every birthday, every milestone. It was so special to me and I look back on those times very fondly. I also became INSANELY close to two of my cousins, one younger and one older.

In 2018, I'm still not fully sure what happened, but my family basically split down the middle. I lost 2 aunts, 2 uncles, my grandparents, and 6 cousins (including the younger one I was close to). This WRECKED me. I knew there was some type of drama going on with the adults that caused this, but I was devastated. I lost half of my family and I had such hard time letting go. My estranged aunts and grandmother called my non-estranged family so many names (drug dealer, wife beater, alcoholic, etc) even though they weren't true. They even apparently said nasty things about me and my brother (keep this in mind). Because of all this, my family kept saying not to worry about them, they aren't worth it, they're evil, etc. But how could I not cling to all those memories? How could I not miss them, even if they did and said awful things? I was also so close to my younger cousin. She was like a little sister to me and I struggled for a long, long time to deal with that. It felt like I was grieving people who are still alive.

In 2020, my nonno passed away. We were ALL in the same room for the first time in 2 years for his funeral. I remember hoping that this would be when things finally changed for the better. That we'd take this tragedy and use our grief as a way to come together again as a family. But when the funeral was over, when we were all standing outside in the parking lot, waiting to see who'd say something first, they looked at us and then walked away. Sealing their feelings towards us. I hadn't seen them since. Also, in my nonnos will, he left everything to my grandma and nothing to his kids. So my dad and aunts got screwed over that way.

Fast forward to now. My parents have been divorced for a year and a half now. My mom claims a big reason why she left is because my dad didn't stand up for her enough against his family. They gave her bad PTSD and caused her to go on medication for depression and anxiety which made her gain weight and ruined her self image. She's always made it clear that if me or my brother talk to my estranged family, she'll "never speak to us again" (but I must note that she's very dramatic). I've always disagreed with how she was treated by my dad's side. She didn't do anything wrong and I'm upset by how they treated her, but at the same time, I miss them. I miss my grandma, my cousins, and all the memories. I'm getting married in October and I often get sad when I realize that side of my family never got to know my fiance. We've all missed out on so much and I wish things were different, yet I also know things can't be undone.

My dad and I were invited to my grandma's 80th birthday party in November (As of writing this, that was over 7 months ago) and I took the opportunity to go even though I knew things would get messy if my mom found out. But I just wanted to go for myself. I've been in therapy for years over this, not being able to let them go and going over so many what-ifs. There was never any closure, which I've had a lot of trouble dealing with. No one got together to talk about anything. My therapist said going could be a good way for me to feel things out and decide if seeing them again is really what I want. So I went and it was weird. It felt nostaglic, yet I felt guilty the entire time. Nothing like the fond memories from my childhood since there was all this tension. Yet I'm glad I went, because everyone was actually really happy to see me and it was nice to see everyone again.

A couple of days ago, my mom somehow found out I went to this party. She BLEWWW up. She's been ignoring me for days, and ranted to my brother about how I "betrayed her" and that its disgusting I went to see them after everything they did and said and that I should never want to speak to them again out of support for her. My brother thinks I messed up and doesn't understand why I'd still want a relationship with that side of the family. And that it wasn't fair to our mom. It just feels so unfair because she's never once stopped to ask me how the family falling out affected me or how I felt about it. She just assumes I'm mentally and emotionally done with them like she is. Yet at the same time, I get why she's upset and I don't agree with what they all did. Idk how to feel anymore. I'm so stressed out that I actually feel sick. She was supposed to take me to my hair trial next week (like I said, getting married in a few months!!) and between this, wedding stuff, and trouble on my fiance's side, and health issues, I'm losing my mind.

So am I the asshole for going to my estranged grandma's birthday party??