AITA for not leaving my estate to my severely disabled brother?

I (33F) received a terminal diagnosis earlier this year. I’ve got a couple of years at best estimate to get everything in order before I won’t have the cognitive ability to do so anymore. I’ve been working with a lawyer and an end of life care social worker to make sure everything will go as smoothly as possible and my wishes will be respected.

I also have a complicated family situation. My parents were never married and my dad died when I was 12 of the same condition I now have. He never married so all his assets were put in a trust for me by my grandparents. My mom married after they broke up and had my half-brother (26M). He is severely mentally disabled. He’s a 7 year old in a grown man’s body and even with all the resources they can get, he’s very difficult to care for and my mom and step-dad barely get by most of the time. They tried really hard to get legal control of my trust when I was living with them, but couldn’t. I help out some when things are really hard, but I feel like my mom treats me like an emergency fund rather than her daughter and my step-dad has been bitter about the financial stuff (my dad was very successful and his family is well off) since even before my dad died. I don’t talk to either of them about my life because they always get passive aggressive and guilt-trippy.

I’ve been trying to talk more with my mom now that I know I’m on borrowed time. We’ve never had a great relationship, but I wanted to try and resolve some of it. She’s been much nicer to me and more concerned since the diagnosis, so I thought it was ok to open up a bit. I didn’t want her to be blind-sided or feel like she has to take on anything since I’ve appointed another family member to be my medical and financial POA, so I talked to her recently about my plans. I told her that I know my half-brother is a full time job so I’m having other people take care of my care so she won’t be burdened. She was thankful for that, but then brought up how she always hoped that I would be the one to make sure my brother was taken care of when she was gone and that in a way I still would be. She would put whatever I leave behind into his care fund and would make sure he knows it’s from his big sister.

This is where I might be the AH. I’m not leaving anything to my mom or brother. I don’t have kids, but I have been the favorite “auntie” to my best friend’s two children for their entire lives. They are the light of my life and I spend a lot of time with them. My BFF and I have been through a lot together. She and her husband have been the ones going with me to appointments and letting me crash in their spare room when I was too sick from meds to be home alone. Whatever is left over after I’m gone is going to the kids for college. I know it’s money that could secure better care for my half-brother and take some of the load off my mom, but I feel like I want to help someone else now.

Edit - Thank you for the best wishes. To answer a couple of questions: (a) I am not at all close to my half-brother, I didn’t really meet him until my dad passed and the 6 years I spent around him were painful and frightening. I haven’t seen him in person in 15 years. It’s not clear that he even understands that he has a sister or what that means and I would be really surprised if he remembered me at all. So, it’s not that I don’t care - I care to the same extent that I would about the welfare of any random mentally disabled stranger - I just don’t have any special interest or warm family feelings about him because we share DNA. I’ve also already put a lot of money into helping my mom stay solvent to care for him, so there’s an argument to be made that he’s already received a reasonable share of benefit in life. As much as my mom refuses to believe it, I would never have accepted legal responsibility for him after her death. His dad has family that could step in if they were willing. (b) I am working with a good team already to get things in order. My estate lawyer is very good at his job and he and my financial advisor are prepared to firmly lock down everything once I’ve made the final decisions about where my assets should go. I trust my executor. So, that side of things and the medical and logistics planning is being well taken care of, don’t worry. I just have to make a few hard decisions that I didn’t expect to ever have to make.

Thinking about some things people have said in this thread, it may be just be time to end the sentence on my relationship with my mom and her family and move on. I’ve always held out hope that she cared in her own way and that things could be better, but I’m really tired and I’m not sure I want to spend what’s left of my time chasing it and trying to figure out whether she can even be trusted to manage any money I could leave to them. The SW offered to set up a few family sessions with a counselor that specializes in terminal family member issues, so I might let that be the weathervane.