How can I find out what condition my dying mother was in when she signed a new will.

My mother had a will in place since 2006, in this will nothing was left to any of her children. My sister took over caring for our mother for a few months till she entered hospice. My mother died Jan. 25, 2019 of colon cancer I was told by my neighbor. My sister had our mother sign a new will leaving everything to her 16 days before she passed. My sister never has till this day has not informed me or our brother of her passing, her condition when she signed the new will, what's in the new will etc. Anything I have learned has been through my neighbor. I am sure there was undue influence. There was no reason to change the original will. My mother had left everything to her friend in California of 60 years. All of a sudden everything was left to my sister in the new will. I want to know my mothers mental condition 16 days before she passed when my sister had her sign the new will. How can I find this out?