How do I talk to my mother about her mental changes and maybe being seen by the doctor for dementia?

My 88 year old mom is showing signs of increasing dimentia. She is also in poor health and is housebound. I have been trying to get her to name me as her attorney in fact, via a durable power of attorney document. I also need suggestions on how to talk to her about her future "care" needs. She is not very receptive to this type of talk, but I am the one who will end up taking care of her and I want to know what she wants.

My mom lives in her own home, but I do all of her gardening, shopping and take care of her needs. She cannot go anywhere on her own so it is up to me to take care of things. I have 5 siblings and none of them are in the area, nor do they make much of an effort to be part of my mom's life or her care. I have asked more than once for them to just telephone her or send her a card to brighten her day, but I have had little success getting them to help. I think some of them resent me for my suggestions. It causes me to get quite sad that they don't care.