Will

My dad and I got into a car accident in June and since the accident he has been having mental issues as his doctor states. I am his only child and we were very close up to 2 months after the accident. This is where he went crazy and assaulted my mom and went to jail. His step daughter, my mom's daughter, the one who he has always hated since my parents got married back in the 60's step in and convinced him to divorce his wife of 45 years, dis own me and his only grandchildren who he cherished and loose their home of 27 years. His step daughter has always hated him. She saw her chance to step in and control his money and will. She is and has always been on welfare. We were told he gave her power of attorney. If he shall die can I fight the will? Can she be held legally for causing all of the misery? Is this elder abuse?
Thank you

If you can show he was not mentally competent then it can be challenged, yes.