Should a mother register a piece of land in her nephew's name in exchange for his financial support of her, given that she has sufficient rights? And is a son obligated to financially support his mother, and what is the extent of that support?

Your mother is not required to register any of her land to your maternal cousin. If your grandmother owns enough for her own expenses, her children are not obligated to spend on her. However, if she is poor, then her maintenance is obligatory upon her son and daughter. The more accurate view is that maintenance should be distributed among the children according to their financial ability, not equally or according to inheritance shares.