Two brothers married two sisters on the same night but they discovered in the morning that each consummated his marriage with the wrong wife
i.e. with the wife of his brother, what is the ruling on this and for how long is the `Iddah (woman's prescribed waiting period after divorce or widowhood) of each wife?
(Part No. 19; Page No. 291)
If the reality is exactly as what is mentioned in the question:
Each one of the concerned two brothers has to be separated immediately from the lady he had an intercourse with, and the concerned ladies have to make sure that no pregnancy has started by waiting for one menstruation period.
The original ruling is that each one of the two ladies has to start her marital life with the person who concluded her marriage contract after waiting for one menstruation period and knowing that no pregnancy has happened. In case there is a pregnancy, the child thereof is to be ascribed to the man who had sexual intercourse with her because it is a suspected sexual intercourse which is different from the prohibited Zina (sexual intercourse outside marriage).
If each of the two brothers agrees to marry the lady with whom he had sexual intercourse and the concerned ladies agree to this as well, each one of the two brothers has to divorce his wife so that the other marries her. Doing so does not need the concerned ladies to wait for any `Iddah or menstruation period because in case there is a pregnancy the child will be ascribed to their father anyway. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.