Is it permissible for the mother to relinquish her share of the inheritance to her son after the two sisters have taken their share of the inheritance that was due to the son?

An adult, mature heir has the right to relinquish their share of the inheritance after acquiring possession and control over it. However, the mother is not permitted to allocate her relinquished share to a specific child; rather, it must be distributed like a lawful inheritance. What you mentioned about the two sisters taking their share from the frontage of the land is not correct, because the frontage of the land does not belong to one heir over another. Instead, the entire estate is to be divided according to the Islamic legal division, taking into account the value of the different portions. You must return the mother's relinquished share to the estate so that it can be redistributed according to Islamic law.