Is selling a piece of gold to one's sister to help her pay off her debt, on condition that she returns a similar amount of gold in the future, considered usury (riba), especially given that the price of gold has increased after the sale?

The consideration in a loan is what the borrower receives. If he receives gold, he must return gold of the same kind. If he receives currency, he must return currency of the same kind. The fluctuation in the price of gold or the change in the value of the currency does not affect this. A loan obligates the return of the equivalent of what was received. Therefore, if the sister received gold, she is obligated to return gold of the same kind, regardless of its increased price. And if she received currency, she is obligated to return currency of the same kind.