1) as per conveyance deed your father is absolute owner of property .
2) your father can execute specific power of attorney in favour of your mother to sell the property , receive sale proceeds , appear for registration of sale deed
3) specific power of attorney has to be duly stamped and registered .
4) your uncle name is not reflected in sale deed . if your father wants to give 50%share in property to his brother your father can execute gift deed in favour of his brother for 50%share of property . on property being sold your uncle can receive 50%of sale proceeds in his name by cheque