we all cope with things in different ways, just because your sister copes in another way does not mean she is wrong. you know your mum is not talking sense when she asks about her parents, it is ok to go along with that, when my gran used to talk of her son and parents,at first i told her she was confused and they had died, she would get herself so upset, then ask the same question again later, all that did was to upset her as she had to go through finding out about their death over and over again. so i went along with her and she was quite content in her own world, she was not right but that did not matter to me. I know itsnot the samesituation but similar. maybe that is what your sister is thinking. you are quite right to prepare yourself for the future, if that is how you best cope on you go. Try and not get too caught up in what others think, you sound as if the pressure is building up inside you, you need to release it. You have been very strong, you also have a good idea whats ahead for your mum, the knowledge about the disease helps us. I think I always felt if i was one step ahead, I would cope better.
Have you spoken to your mum, does she know how bad the situation is, if so how is she feeling? She must be so scared, just like you. you might be able to ask her, what her wishes were, then the decision would be hers,not yours, it might take some of the pressure of you. You are certainly not a horrible daughter, quite the reverse you want the best for your mum.