There are really only three reasons the sister would not donate at this point,IMHO.
1.A contrived politically oriented reason:Use this situation to write new laws compelling forced organ/cell donations for best transplant matches.
2.Personal family reasons, that are none of our business, but reflect very badly on the brother.
3.Personal family reasons, that are none of our business, but reflect very badly on the sister.
Not enough information is available to decide which of the above three apply, at this point.
I can think of one other possible reason:
4) Sister has contracted an illness (possibly sexually transmitted) which she doesn't want to spread to her brother, and is too embarrassed to tell him about.
No, we don't know the reasons, of course.
All I can say is that I can easily conceive of having such a bad relationship with a brother or sister that I never wanted to see them again or have them in my house, whether it was my fault or hers or some combination of the two. I think I would still be willing to donate bone marrow to save that person's life, however. Maybe we'd never speak to each other again even after that. But it still seems wrong to me to deny someone the chance to live.
A possible fourth reason is that she really enjoys playing God... having the power of life and death over another person, or having the wife beg. Once she donates, she's just another donor. Hold back and she is like a god.
It's much more simple. She is, in effect, murdering him.
And that's evil.
Unless she fears for her life at his hands, there is no excuse I can think of. A mitigating factor, perhaps. But no excuse.
If this woman doesn't change her mind, soon, everyone in the community should make her life miserable -- for the rest of her life.