Whether your mother's marriage with another religious person is not necessary. All the legal heirs (including your mother) of your grandmother have equal rights over the property. So consult a local advocate to file a case for partition.
How that matters, your mother has full right in her ancestral property, that is right inherent , just file FIR against othe fraudisters. Make also publication in news paper about the property that no one may puchase it being unpartitioned and in dispute...
The simple fact is that your mother might have left Hinduism and the religion can not desert her. Has she converted as Muslim ? Even then Hinduism can not desert her.
ONCE A BORN HINDU, ALWAYS REMAIN AS HINDU UNDER LAW, AND THIS IS NOT THE SAME WITH HER CHILDREN.
The property of your grandmother shall devolve upon her children, including your mother, as she (grndmother) was a Hindu.
The post involves complicated facts vis-a-vis law applicable.
It is better to seek professional service of a local prudent lawyer for appreciation of facts and guidance.