Because of rain, my mother's grave is going to disappear. If it completely disappears, then somebody else's body will be cremated on top of it. How can I identify my mother's grave? If another body is cremated on the same place, then how can we visit the grave, and will my mother be able to hear me when I talk to her?

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 believe that the matter is not difficult. We have previously clarified to you in fatwa 90624that you may place a mark, such as a stone, over your mother's grave to be able to identify it. According to the Sunnah, the grave should be slightly raised above the ground with a prominent mound over it in order for it to be recognized as a grave. Sufyan at-Tammaar said that he had seen the grave of the Prophet and that it was humped. [Al-Bukhari] Humped means raised above the ground by a hand span like the hump of the camel.

Cremating a body in the same place where your mother's grave is located does not forbid you from visiting her grave.

As for your question as to whether she can hear you, the scholars held different views about whether the dead hear the living. The preponderant view is that the dead hear the living  in general. This does not necessarily mean that the dead can hear the living all the time; they might hear in a given situation and not hear in other situations, as was underlined by Shaykhul-Islam Ibn Taymiyyah . This means that your mother may be able to hear you sometimes but not all the time. For more benefit, please refer to fataawa 84867 and 84963.

Allaah knows best.