Hi Lucky,
I had a different situation, my NF had terminal cancer and I had a NM as well.
In your situation, I would follow a lot of what is said here.
Focus on you and your health, and the health of your immediate family as a priority (partner/children if this is the case). From what you have written, it sounds to me like your father is the last person who would want you or yours to suffer for him?
Focus on what you want to give your father and what you want to remember as your last days with your father if he is terminal. (I am so sorry if this hurts or sounds harsh – I was in such denial when my father died, I couldn’t focus on what was happening let alone what came after – not that you are there, just my experience.)
You decide how you want this time with your father to go, and don’t let anything interfere with that.
To borrow Hopsy’s analogy, imagine your mother as a gale-force wind that you have the strength to stand tall in (and you do) while that wind splits harmlessly to either side of you.
IME, ignore her as best you can; if you can’t just say “OK M.”
I’ve found it takes the steam out of them because in their mind you are agreeing with them.
Even if we don’t agree – ultimately it gives her what she wants without compromising yourself, and hopefully, the situation won’t escalate into something you may not have the energy to deal with right now.
IME, if in her mind, you are ambivalent to her need, she will come back for more; if you directly argue, she will become voraciously addicted to you and come back for much, much more.
If you give her the guise of agreeing, she will leave you alone (that was my experience in dealing with Ns until I went NC, even though it doesn’t feel honest to say “OK” because everything is not OK – and that is how I felt – it got them off my back – and I needed that – the situation was so stressful in and of itself – dealing with an N on top of it – no words!).
If you need to, throw her an OK bone that gets her away from you (I have found it is the best you can do with dealing with an N if you don’t want them to go nuclear – unless you go NC – and even then they can go nuclear!)
One exception to this, if your NM becomes dangerous to you/yours or your father (and I had this happen when my father was dying); then, unfortunately, it is time to step up and handle it as best you can.
She is nothing more than a palm-full of sand that when your fingers open, she falls through.
If she is anything like my parents, this time will become her time.
It is not her time.
It is yours and your relationship with your father.
I am so sorry. I know how difficult it is when one we love has terminal cancer.
Much, much love and strength to you,
Peace