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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
There is no need to worry about what happened as it does not mean anything. We ask Allaah to forgive your mother and all the dead Muslims. You should supplicate Allaah to forgive her and have mercy on her, as Allaah Says (what means): {…and say: "My Lord! Bestow on them Your Mercy as they did bring me up when I was small."} [Quran 17:24].
For more benefit on the deeds that could benefit a person after his death, please refer to Fataawa 88956.
Finally, we hope that the sickness and suffering which your mother experienced before her death will wipe out her sins and increase her status.
Allaah Knows best.