Hello - although I am not able to provide you with very much information, I can certainly sympathize with your situation. I too am facing a change in the lady I care for and wonder just how to deal with it. She was previously kind, polite and fairly even-tempered, but now lashes out at me over absolutely nothing at most times or things she has contrived in her mind at other times. She has a daughter that lives close by but who stays away from taking care of her mother, choosing instead to hire caregivers for her, and who just doesn't understand when I explain to her about the changes in her mom's attitude and behavior. I know this woman needs to be examined by her doctor or by a neurologist, but I cannot seem to convince her or her daughter that medical care is needed at this point.
I am sure that I did not answer your question, but perhaps it will help you to know that you are not alone in your problems and that others are struggling with the same difficulties. One thing I would like to comment on about your particular situation - it sounds like you are starting to become overwhelmed with handling this problem alone, which is certainly not fair to you since you indicate that you have a brother and a sister. Perhaps a heart to heart talk with them, telling them exactly what is going on and how you feel about it, may help? I suspect that their being "in denial" because they don't talk to her that much is actually being unaware because they don't spend the same amount of time caring for your mom as you do, and they may be happy to leave the lion's share of that responsibility to you. It sounds like you have already been assuming the role of the primary caregiver for your mother, and since it is probably unchartered territory for your siblings I would bet that they see no reason to change things as they exist now. Let them know, loud and clear, that caring for your mom is something that all of her children should be involved in, and that you need help in deciding how to handle changes in her personality, strategies on talking to her and convincing her to see her doctor, etc. Now I wish my own advice would be helpful in dealing with my lady's daughter, but your situation is somewhat different since this is your brother and sister. I wish you lots of luck and hope that you can work something out with them so that you won't feel overwhelmed and ultimately resentful against them for not helping.
I learned a long time ago that while caring for an elderly person, the very first thing you need to do is to take care of yourself. If you are physically, mentally or emotionally unwell, you really won't be of very much help to anyone else. Your frustration will continue to increase if you don't address it right away, which won't be good for you or your mom, so prevail upon your siblings before this problem grows much bigger for you.