Is the marriage valid if it turns out that the wife is not the one originally contracted upon?

If the offer and acceptance for "A'hud" took place, then she is lawfully your wife. There is no sin upon you for having intimate relations with her sister if you were ignorant of the situation. You must now immediately separate from the woman who is with you, as you did not contract marriage with her.

Regarding the child, her lineage is established as yours. If the woman with whom intercourse occurred was ignorant of the situation, she is entitled to the dowry of her peers (Mahr al-Mithl), and you may seek reimbursement for the dowry from the one who deceived you.

If a woman is presented to someone other than her husband and he has intercourse with her, the one who engaged in intercourse must pay the dowry of her peers (Mahr al-Mithl), and there is no prescribed punishment (hadd) upon him, and the child's lineage is established. Ahmad said concerning a man who married a woman, and her sister was brought to him: "She is due the dowry for what he attained from her, and her sister is due the dowry," and "he should seek reimbursement from her guardian." This ruling should apply to a woman who was ignorant of the situation or of the prohibition. However, if she knew that she was not his wife and was unlawful to him, and she allowed him to have intimate relations with her, then no dowry is due to her because she is an amenable fornicatress.