My mom is 86 and shows signs of memory loss and has gotten real negative, bitches and complains no matter what you do. She won't tell any of her children her funeral plans, won't let us know any her medical info or who to contact for her insurance. None of her business if something happens to her. What are we suppose to do? She has 5 children and none of us know any of her important info.
As to her funeral, she may not want to talk about it because it only brings home the fact that she is nearing end of life. It is an uncomfortable topic for many people. Or, maybe like my mom said, "What do I care what kind of funeral I have? I'll be dead." My mom truly doesn't care if we go traditional or with cremation. And as she said, the funeral is for us, not her.
Your mom's bitching and complaining may also be a sign of dementia. She may be feeling overwhelmed with getting her paperwork in order. She may not be able to concentrate on it all and that frustrates her so it comes out with negativity. One of you just needs to calmly sit down with her and help her go through things. Find out where her important papers are. Help her pay bills. Just help her go through her mail. These are all things that are "too much" for a person when their memory goes and/or dementia sets in. As this sib slowly takes over these tasks, I bet she will eventually be quite grateful to turn it over to that sib. My mom was very happy to not have to worry about all those details. Persevere and good luck.