This is like comparing apples to oranges. That man didn't make the sisters a favor. He married them for his own desires and nothing righteous about it. If you marry a woman and that woman has brothers and sisters. Then her family becomes your family. The sister is forbidden fruit to THAT man. Why not just promise to care for his sister in law like a brother cares for a sister in desperate need of that help? This is not a church issue, this is a society issue. In his country, was this choice the norm or did he decide on the exception to the norm?
you are thinking like a westerner. a woman who cannot bear children is seen as a burden in their culture. three things that should never be delayed, according to their faith: prayer time, funeral preparations, and marrying off a young woman to a suitable partner. and since she was not considered marriage material, the family jumped on the chance to marry her off.
her family treated her like a burden, and even if he had sent them money to take care of her, it probably wouldn't have done her any good. so he married her and set her up with a monthly allowance, and took her out shopping for stuff anytime she needed or wanted anything. without marrying her, supporting her like that would have been practically impossible.
it was the fertile sister that convinced him to treat her barren sister as a regular wife, after he had already been supporting her in her own house for a year and a half. until then, he had never consummated the marriage to her because he didn't want to cause strife between the one he wanted to marry and her sister, who he agreed to marry so that he could take care of her, so as to keep the one he wanted to marry happy.
his choice was an exception to the norm. practically unheard of. in the end, he fell in love with both of them.
remember, we are talking about places where they still do honor killings...