Praise be to Allah.
It seems that your mother in is need of care and compassion, and at the same time she seems to be feeling some nostalgia for the past, by wishing to go back to her former corrupt religion. The former stems from her losing her position through the divorce which has taken place, so you and your brothers and sisters must make up for what she has lost to reduce the pain and bitterness of the humiliation she has suffered through this divorce. Most of the responsibility will fall on your shoulders because you are the closest of her children to her, as you say. You must enjoin your brothers and sisters to fulfil their duties of caring for her and looking after her, by speaking kindly to her, consoling her gently, serving her, taking care of her affairs, spending on her, visiting her, keeping in touch and upholding the ties of kinship, until things settle down for her and she feels better.
With regard to the second issue, her nostalgia for her former, corrupt religion, you must deal with this problem by advising her and making the effort to explain the seriousness of apostasy and returning to kufr. Tell her about the hadeeth of the Prophet (peace and blessings of Allaah be upon him): “There are three things, if a person has them, he has found the sweetness of faith: if Allaah and His Messenger are more beloved to him than anyone or anything else; if he loves another and does not love him for any reason other than for the sake of Allaah; and if he would hate to return to kufr after Allaah has saved him from it as much as he would hate to be thrown into the fire.”(Reported by al-Bukhaari and Muslim. This version was narrated by Muslim in his Saheeh, no. 460).
Alarm her with the aayah (interpretation of the meaning): “Verily, those who have turned their backs as disbelievers after the guidance has been manifested to them, Shaytaan has beautified for them (their false hopes), and (Allaah) prolonged their term (age).” [Muhammad 47:25]. Explain to her the fate of the apostate as described in the Qur’aan (interpretation of the meaning): “…And whosoever of you turns back from his religion and dies as a disbeliever, then his deeds will be lost in this life and in the Hereafter, and they will be the dwellers of the Fire. They will abide therein forever.” [al-Baqarah 2:217].
If you try your hardest, you will not be to blame if something that you dislike happens after that. But whatever the case, you must not carry out any last wishes or will to burn the body after death (cremation), because this is something that is viewed as abhorrent in Islamic sharee’ah (see Question #675). We ask Allaah for guidance and steadfastness in Islam, and for a good end, for your mother and for all the Muslims. May Allaah bless our Prophet Muhammad.