AITA for not trying to help my moms marriage

A little backstory here. I will be referred to as “Posters brother” or “Brother” from now on. So my Dad married my mom really young and before that, my mom was in a really bad relationship, she was beat up and burned and was lucky to be alive. So she married my dad to escape that ex boyfriend. My mom has admitted she was never in love with dad. In fact, she loved the other boyfriend and was mad at him for not saving her and being a piece of shit. She hates him for beating and burning her but for the rest of her life she will be in love with him. Anyway she married my dad when she was still 19. She hates the fact my dad was never good in the bedroom. She wanted to leave him for a long time but didn’t for us (me and brother) until she finally did when I was 14 and she married this old man. Brother and I stayed with dad and mom stayed with her husband for a while but now she lives by herself. She tries to pretend she’s happy with her husband George and she’s not. George is 72 and is not that good in the bed either and she is bored out of her mind. Anyway to the main point.<br><br>My grandma on my moms side is visiting and will stay for three months. She hates my dad and always has. She has this crazy idea that if my mom and dad get back together, she will be happy and so will we. Grandma has been trying to get them back together and it’s getting annoying. One night my grandma had this “brilliant” idea she was going to have dad over for dinner and then after dinner she would make them both go to the same bed to “talk” for a while. Well, guess what? It worked. Dad and mom hooked up for the first time in years and now my grandma is smug AF. Well, brother and I are both against this. Dad was really heartbroken when mom left him and if they get back together, that will mean she has to break up with George (meaning she has to leave us to live with dad again). My mom has also threatened to cut brother and I off if we don’t accept dad and have said she might even not pay for our college anymore.<br><br>I think this is bullshit. I think my grandma is being selfish and I hate my mom. I don’t want to lose dad for good. Brother and I are trying to just ignore them and not help them get back together. Anyway here I am. AITA for being passive about this and not trying to warn dad about what’s happening?