I lost my stepmom to cancer over a year ago. All I ever want to do for declining cancer patients is to make their day a little lighter somehow. It's a tough one, I agree. I like how OnTheWayOut strikes a balance:
If you think it's best for her health to not dash her hopes, and you can not get caught in a promise to do something toward reinstatement, then you could refrain from telling her you have no intention of going back. If she wants promises, then you have to say NO.
Maybe because I have never been a Witness, I am able to comfort JW's with the words that will make THEM feel better. For instance, when my JW husband's mother passed away, I reassured him that a loving Jehovah would surely resurrect her to the paradise earth, she was such a lovely woman her whole life (not a JW by the way). The congregation, on the other hand, "comforted" him by reminding him to stay faithful in service so he could see her some day.
It must be tearing your mom apart imagining that she will be in paradise, alone, without her children. Does your mom still talk to you? If she does, perhaps you can comfort her with the thought that Jehovah's love and Jehovah's justice is perfect, far better than any man's. Even imperfect elders. Surely Jehovah can look at the hearts of ALL your siblings and give them a chance to join her in paradise.
Or perhaps you can ask if Jehovah speaks to the hearts of the pagans in the deepest China, where no literature can penetrate. Does not Jehovah speak through creation? (Acts 17:22-32) If Jehovah is everywere, reading all men's hearts, not just in the Kingdom Hall, couldn't he read YOUR heart as well and find forgiveness, even if the elders haven't?
Just a few thoughts.