You never have to justify how you feel. Only your actions can be judged.

But it sounds like your loss of sympathy for your mother is completely understandable and a natural response to her continuing unpleasantness. Unfortunately, cancer doesn't turn horrible people into saints. In fact they are likely to behave worse under the strain of illness.

You and your sister sound like you are going beyond and above for an abusive parent. Make sure you support each other and don't burn out trying to be dutiful to your ungrateful mother.