Must the sister return the value of the gold at the time of its sale or its current value, and is it permissible to demand gold of the same kind, including its manufacturing cost?

It is permissible to demand a like amount of gold in terms of weight and specifications, if possible. Otherwise, the higher of two values should be given: its value on the day it was sold, or the price at which it was sold. This is because a loaned item is guaranteed, and any decrease in its value is covered. If it was sold for more, then the lender is entitled to the price at which it was sold, because its price is the property of the lender.