I'm thinking of adopting my sister's cat
Why do you want to adopt your sister's cat?
She has a disease and it is quite a burden to take care of a cat.
If she chooses to let you adopt it then that is fine. But you cannot choose for her. It is her pet and it is her decision.
I don't care that she's going to be upset, I will do what I want.
It is not your position to tell her what to do. It is up to her, you cannot do it for her.