All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, please accept our condolences on the occasion of the death of your mother, and we ask Allaah to forgive her, have mercy upon her and admit her to Paradise.
The fact that your mother was not able to keep her Hijab on when she was sick and disclosed part of her body does not mean that this is a bad end of her life, and she is not sinful for doing so, Allaah willing.
Therefore, you should not worry about this matter; rather, you should endeavour to be kind to her after her death by supplicating for her, seeking forgiveness for her, being generous to her female friends, and keeping ties of kinship which are dependent on her.
For more benefit, please refer to Fatwa 87019.
Allaah Knows best.