What is the ruling on divorcing one's wife if it was conditioned upon the sister returning to her husband's house, and the sister did return before the brother's second marriage, knowing that this oath was sworn twice in two different sittings?

You made a mistake in conditioning a divorce upon your sister, and this is a conditional divorce that takes effect when the condition is met, according to the majority of scholars. Some scholars are of the view that if you intended a threat or an urging and did not intend divorce, then you must offer an expiation for a broken oath (kaffarat yamin). However, if you intended divorce, then the first divorce has occurred, and the second divorce does not occur except after a return (to the marital relationship) or a new marriage contract.