My dad was taking care of his father, his sister was always passive. My dad got a stroke, how can we oblige his sister to take care of her father?

Anonymous
So, my father and my mother have some conflicts with my father's sister- my aunt. They have a father, who's 95 and needs daily care. My father takes care of him, changes his bed, and his clothes, and bathes him, it's horrible to look at it, he's a big career as well and lots of things to worry about. He has us, 3 full-time students, studying away from home. Whereas my aunt is a single woman, working and never visiting her father, only sending him medications. They refuse to send him to the center for old people, because of some moral reasons. My aunt's behavior made my mother really angry and they are not on good terms, they don't talk at all.
My father got a stroke a few days ago, because of exhaustion and the reason was that he could not take good care of himself. He's in a difficult situation, he can't even move, we'll have to take care of him - whereas his father is in a much better shape but he refuses to do simple things like going to the bathroom to urinate. He's a difficult person.
We're facing financial issues currently as well. I'm looking for a higher salary job + studying, same with my siblings (one if them is a minor). My mother will have to take care of my father who can't even move. But my aunt now assumes that now we'll have to take care of her father. She tries to give this responsibility to us, which I and my siblings are not willing to take. We have to work to bring money to home, we won't be at home and sitting with an older person daily. It seriously affected our father, who was not able to handle so much.
How can we talk to our aunt? We're nervous because we don't want to cause conflict. We still want to maintain a good relationship with her, she's also a medical worker. But we want to make it clear that we won't be able to pay/become caregivers for our grandfather and she has to take responsibility for her father (She has not even seen him for 5-6 years, I don't know what's wrong with her). We need your help.
My dad was taking care of his father, his sister was always passive. My dad got a stroke, how can we oblige his sister to take care of her father?
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