OP you were right to stand up to her.
You should not fear your husband.
Your husband should not have put you in this position.
Do you have kids to care for, do you work? If so then your time is needed else where. If your dh wants to go and help his mum, he can. You can get on with the rest of your life.
Your husband has put you in an intolerable position.
I have lost both my dad and my mum. I loved them dearly but my first priority is to my kids, and then to dh. My parents in law are lovely people. But I would not live with them.
'It is hard for DH to move out as he feels he should be around his DF until his last breath.' If he ends up sacrificing his marriage to you in order to wait around his father and help his mum, he will be very foolish. If his father really loves him he will not expect his son to be around for his every breath.
It seems you may be from a culture where women are expected to wait on others and fear them, if you are in the UK, you know that this is not how most women here behave or want to behave.
Your husband must be very sad at the prospect of loosing a parent, my dad died while I was in my thirties, it must be very hard if your husband is even younger.
However, he is a grown man, with a wife and he needs to not sacrifice you to his family's needs.