Prohibitions For Marriage
Every directive is based on a cause or illat. When that cause is found at other places the directive is extended. The cause of prohibition of marrying close relations is to avoid rivalry. Prohibition of marrying a woman and her daughter and of marrying two sisters in one nikah, is to avoid these close relations develop rivalry. A woman and her niece is as close a relation as woman and her daughter. Thefore the prohibition will extend to marrying a woman and her niece in one nikah.