I am so sorry you have to deal with this type of thing along with the grief of losing a loved one.
My mum died of cancer 3 years ago (RIP mum) and was brought up as a church of england follower, walked away from her religion when she was about 16. My sister is a born again evangelist, as his her husband and children. She cared for mum in mum's final stages and spent a lot of time 'converting' mum back into the christian faith (mum was into buddism in the last 20 years, not practising, but as in interest). She scared my mother into returning to her faith, as it was my sisters beleif that my mother would not be saved unless she went back to god. She played on my mum's fear of dying. She read the bible to her every night, as mum was not strong enough to say no, or, in the end to think for herself. She had a priest/vicar visit her and she made peace with god'.
When she died, my sister ensured her funeral etc was religious.
She destroyed the family as we would not indulge her, or participate in contributing to the immense fear my mum already had of dying.
I have not spoken to her since the funeral, for this and also other reasons all wrapped up with this. I think, if she had less involvement with her care, and the chosing of her care etc I would have just accepted her views. If mum had been in a hospice, as I beleive she should have been, it would not have been so intense.
I do not know why she was like this either, so insistent, I guess she really did beleive that my mother would not be saved and would not be at peace if she did not 'repent her sins'.