After cancer

Doctors discovered a massive brain tumor on my sisters frontal right lobe in 2012. She was immeadiatly admitted into the Neuro ICU and operated on the next day. She then spent 10 weeks at MD Anderson recieving chemo and radiation every day. She was only given 3 years to live and miracously the tunor completely disappeared. But now almost 4 years later the affects of her cancer are worse than ever. She basicly has a mind of a slow 10 year old. We expected some loss in cognitive abilities but it seems to be getting worse and worse. She doesn't believe anything is wrong (she even Denys ever having cancer sometimes). My once hardworking, loving, compassionate sister now can't hold down a job for more then a few weeks, and is extremely irrational. We recently found out she hasn't paid any taxes the last year and is in trouble with the IRS but refuses to let our accountant help her. Things like this go on everyday. It makes it worse that she has a 4 year old son who was born right before they found the tumor, her and her ex have gotten divorved since because how bad she has gotten. Please anyone that can help, me and my family are afraid if we try to get power of attorney or anything she may run with my nephew. We are also afraid that if she loses the custody she has of my nephew she will hurt herself because she gets in depressive states.