What should the eldest son do with his mother who has recently been divorced and lives alone, refuses to live with her children, insists on having her body cremated after her death, declaring her return to Hinduism, then Islam, and so on and so forth?

Your mother needs affection and care after her divorce, so you and your siblings must compensate her and help her psychologically and financially. As for her longing for her first religion, it should be confronted by admonishing her and clarifying the danger of apostasy, reminding her of the Prophet's saying (peace be upon him): "There are three qualities, whoever has them will taste the sweetness of faith...", and frightening her with the noble verses about apostates, such as the Almighty's saying: (Indeed, those who reverted to their backs after guidance had become clear to them - Satan enticed them and prolonged hope for them) and (And whoever among you reverts from his religion [to disbelief] and dies while he is a disbeliever - then those have become worthless [all] their deeds in this world and the Hereafter, and those are the companions of the Fire; they will abide therein eternally). And do not carry out any will to cremate her body after death.