What is the Islamic legal ruling on a sister demanding the value of a piece of gold she gifted to her brother 20 years ago? And is it obligatory to return its value at the time of the gift, its current value, or the amount she is demanding?

Your sister does not have the right to revoke her gift, and you are not obligated to return the gold piece or its value, according to the hadith (definition): "It is not lawful for a man to give a gift and then take it back, except for a father in what he gives to his child." There is also the consensus of scholars that it is not permissible to revoke a gift given to a mahram (definition) (unmarriageable kin) relative, except for a father in what he gives to his child.