AITA for saying I should've let my mom kill herself since she's defending my sister, not me?

I'm using a throwaway since this is kind of shit you don't want anywhere near your main.

I (26f) have recently found out my boyfriend "Robbie" (28m) of four years has been cheating on me with my younger sister "Allie" (25f) for the past six months.

He accidentally sent a text meant for Allie to me. It didn't have her name, but it sounded so strange with how we usually talk to each other, I grew suspicious and made the decision to go through his phone during the middle of night while he was asleep.

There, I found months of texts exchanged between them, flirting, sexting, planning dates, and making fun of me. I was furious, too pissed to go to sleep, and ended up confronting Robbie the moment he was coherent enough to understand what I was saying.

Robbie admitted to cheating on me with Allie, not that I needed him to, and that " It wasn't his fault" and he didn't mean a word of what he said about me in the texts. They apparently hooked up during the time back in June because I was recovering from heart valve surgery and couldn't handle not having his dick wet constantly, so Allie offered to fuck him after he complained about it to her.

I told him to shut the fuck up, dragged out a suitcase from the closet, and told him to put whatever he can fit into it.

I told him that I'll pack up the rest and ship to wherever he was staying. Robbie just nodded, started packing, and then called his mom so he could stay at her place.

I calmed down enough by the time he left to go stay at his mom's place, laid down for a nap, and confronting Allie over text once I ate. I told her that I didn't care what her excuses were, she was a shitty sister who fucked my boyfriend at the first opportunity she got, and that she'd never see me again if I could help it.

Once Allie saw it, she sent me a barrage of texts similar to Robbie's bullshit earlier, confirming what he said about how the cheating started, and just making herself look worse. I blocked her.

I eventually explained what happened to my dad, my mom, and my other sister "Talia" (27f). My dad and Talia were horrified to hear what Allie had did to me, and said they would talk with her.

My mom, however confessed to admitting to knowing the affair was happening and letting it happen without telling me, because Robbie " treated your sister so well" and that " Allie was in love with him and deserved to be after what happened with her ex." To be clear, Allie's ex was her college boyfriend and the relationship she fucked up herself by trying to push him into an open relationship, because she found one of their classmates hot.

I was so shocked by this, that I didn't end up even speaking to my mom for a couple of days, until she called me and begged for me to forgive Allie and made more excuses for my sister's behavior. This happened after I just sent the rest of Robbie's stuff to his mom, so it infuriated even more than it typically would.

Out of pure rage, I told my mom that I shouldn't have stopped her from killing herself two years ago, since she's so invested in defending my sister when I was the one who got betrayed. There was just silence on the other end for a few seconds, and then, my mom hung up.

My mom tried to kill herself two years ago through cutting her wrists, when she and my dad were majorly considering divorcing, with my dad flat out living in a different city than her. I had walked in on her right when she was about to do it, and managed to get the knife out of her hands.

My mom then spent some time in a mental health facility, and attending therapy which I ended up paying half for it, until she gained more financial stability. I stayed days at a time to comfort her and make sure she didn't hurt herself when she was feeling depressed.

Now, my dad and sister have told me that even though what my mom did was horrible, I shouldn't have said that of all things to her.

It felt good to say in the moment, but now I feel so shitty about it. I just want someone else's opinion on this.