My wife and I went through similar manipulative shit when my FIL died. Several things stand out to me here.
1. She has a brain tumor and while the docs might be telling you it shouldn't be affecting her personality/reasoning abilities, that most likely *is* happening.
2. The second thing is that she's months from death and even a healthy mind is not going to handle that well. Suicide ideation is common. Refusing to accept limitations (WTF do you mean I can't drive?!) is also common.
Ultimately, you are only going to be able to help her if she allows it or you have the support of whomever holds her power of attorney (POA). Without her agreement or a POA, the docs won't listen.
You could hire an attorney and fight your family in court to gain control of her care, but you will probably not succeed and will alienate your brother (though I don't think you'll care) and your father (or her husband, not sure if these are separate people).
I do NOT envy you this. Deciding to continuing being involved and constantly fighting will be Hell until she dies (and for some time after). If you walk away you have and entirely different kind of guilt AND the disapproval of the rest of your family.
I would suggest that *you* find a professional to talk to to help you through this.
