Helping Handicapped Distant Relative with banking needs

My Mother's First Cousin( Father's Sister's Daughter) aged 70 is living alone . Till last June 2024 was handling her banking needs on her own. Her class one heirs are passed away. Class two heirs like brother and sisters daughters and sons exist . One of them is in US . Others are in India . None of them visits her or talk to her. My Mom does not know their whereabouts or personally know them well or have any influence on them. This first cousin of her developed vascular dementia since june 2024 and has lost ability to write or read or properly understand words we say. She has other cognition. She walks . understand purchasing , money etc. With the help of some one can go to bank. But she understands withdrawl and deposit of money in bank. She can sign her name.My mom fears if she looses ability to sign how she can withdraw and use money. A) So is it legal that my mother adds second joint name in her sb account and deposits so that whenever needed money can be given to her B)Any permission or legal documentation required from court to do that. C) How to handle in case of her death the accusitions of misuse of funds from class 2 heirs which she believes will show up after her death.