Can married daughter legally gain guardianship over her parents if they are physically and mentally unable to care for themselves?

My Uncle is 80 years old and he is mentally incapacitated from past one year or so. Physically also he became weak and cannot travel. He is a retired professor and he gets his pension. His unmarried daughter(not financially independent) looks after him. His wife(legal nominee) recently expired. As his money is in bank, it is difficult for his daughter to take care of her father and meet monthly expenses. What options does she have to get power of attorney/legal guardianship to access his financials and take care of the family. He has other 2 children, and they don't have any problem to have their sister get access to their dad's account. Also, how long will it take if they have to get the legal guardianship thru court.