Going Through a Tough Phase at Home and Could Really Use Some Advice

Assalamu Alaikum everyone. I'm using a throwaway for obvious reasons. Things have gotten really hard back home while I've been away studying in another city. My parents have been really stressed lately. Every time I visit, I see my mum crying a lot, and it's because of how my dad has been treating her. She told me he's been neglectful, saying hurtful things about her weight since having kids, and other stuff that's too painful to even write down. He hasn't even given her the mahr properly, and there are whispers he might have another wife secretly. It's all breaking her heart. Both of them keep coming to me, their oldest son, to vent. My dad complains she doesn't respect him and undermines him. The latest hurt is that my dad is planning to go for Hajj again-without her. She's always dreamed of going together, and he just announced he's going alone. He already got his visa. My mum said if he goes, she'll leave. I tried talking to him, but he keeps avoiding me. I have three younger siblings; one is grown and on their own, but the other two are still in school. This tension isn't good for them at all. I have to return soon to finish my master's and can't stay long. I'm thinking of bringing my brother, who's about to start high school, to live with me to get him out of this situation. But then there's my mum-she's completely devastated. She's given so much and feels unloved, and my dad won't listen. One idea I have is to write him a letter, since he hates face-to-face conflict. I want to include advice from the Quran and Hadith, anything that might help. But I'm unsure, since he's funding my education. Writing this, I can hear her crying downstairs. We're all reaching our limits, especially her. Any guidance would mean a lot, JazakAllah Khair.