Oh OP, I feel for you, it's so unfair. How could any mother leave her ill daughter and disabled child needing financial help when leaving their substantial assets entirely to a son who definitely doesn't need it all. No decent mother does this, unfortunately, like I had, you have a horrible, horrible mother.
In my case, my father had made their money, then lost most of it setting up my narcissist brother in business. They almost lost everything but my father managed to make a reasonable amount before he died, leaving my mother comfortable. Since then, her "difficult" behaviour got worse and worse and eventually I realised she was a covert narcissist and she was so abusive I had to walk away. Letters to her accomplished nothing and she lied to everyone about me.
When she died, she gave a few bequests including a token sum for me, and left the rest to said brother. There is one relative who is still in touch with me, and she know mother made a new will just weeks before she died. But she told people that her son made her do it. There are messages from him to people saying she was dying, very confused, weak etc.
So I engaged solicitors to see if I had any case. It turned out her solicitors came to her house, satisfied themselves that she was compos mentis, and noted that "despite her efforts I had staunchly refused to engage" and her heart was broken etc. etc. My solicitor advised that I could pay a lot for a higher legal opinion, but she felt strongly that I would be highly unlikely to win anything; people are allowed to leave their money to whoever they want, even though it can be completely immoral. The token amount she left me would show that she had "provided for me".
Unlike your case, there wasn't a huge amount involved, but it would have made a difference to my children so I felt very sad about that, but ultimately I wasn't prepared to throw good money after bad so I didn't challenge the will. My brother has pissed off almost all the family since, he is a horrible person and will die miserable.
I suspect you would receive the same advice as I have unfortunately.
The only thing I can suggest is to write to your mother and ask her to consider your needs and your child's needs, that you regret you don't have a closer relationship but hope "as a mother" she will leave you some consideration to help in your challenging life, and help enable you to think of her kindly - or words to that effect. I reckon you've nothing to lose (and understand about your brother not being willing to help, it's just horrible how some people are). Good luck.