So sorry to read of your family treatment.
You would know your mother better than us. If she really wants to do as Watchtower commands and if visiting her is out of the question, then so be it. I would just be sure which is better- visiting and upsetting your brother or doing as asked. Your mother is ill and might actually appreciate visits. But you should know for yourself.
Were it me, I would have to be sure to keep abiding by that decision even if she outlives her money, even if she needs greater care, even with her eventual funeral. People will expect you to come through with aid/money after treating you like dirt. I would not respond. You know that your brother will have nothing to do with you either way.