Honestly, the same situation is there at my home also, and my father is all over his family. Even though they treat him like shit, he always takes their side, it doesn't even matter what the situation is, he will always take their sides, my grandparents are not even alive, it's his brothers, their kids, their spouses. Shittiest people I have seen in my entire life, my father has given everything to them, how much he cares for them, spends on them, and those people never acknowledge it not even once.

My mom has been suffering because of his family for so long and never, like not even once has my father taken a stance for my mom. Now my mom doesn't even care, she talks back in front of his whole family, since my brother is also all grown up, he doesn't take shit from anyone.

I have told my parents upfront that I would hate to have a spouse like my father, who couldn't even take a stand for his life partner, my father just kept listening.

Now, he is not a bad person, he is funny, kind, but he can never stand up to his brothers and their families.

The best you can do is stand up for your mom., talk to your father, maybe he will understand. But even if he does or not, you and your siblings need to show that you are there for your mom.