A Muslim married a virgin
and then traveled before consummating the marriage to get the rest of the Mahr (mandatory gift to a bride from her groom) and money for other preparations. When he came back to the girl's father, he expressed his desire to marry the girl's younger sister. So the father agreed once he divorced the oldest girl. The man divorced the girl and married her sister the same night. What is the ruling on doing this?
If you are engaged but not married to the older sister, (Part No. 18; Page No. 240) then you decide to marry the younger one, this will be permissible and your marriage contract to her is valid without having to divorce the oldest sister simply because you have not married her. Furthermore, if you have concluded the marriage contract with the older sister without consummating the marriage, it will be permissible for you to divorce her and marry her younger sister at once, as there is no `Iddah for a divorcee whose marriage has not been consummated. Allah states:
O you who believe! When you marry believing women, and then divorce them before you have sexual intercourse with them, no ‘Iddah [divorce prescribed period, see (V.65:4)] have you to count in respect of them.
However, if you have concluded the marriage contract with the older sister and consummated the marriage, it will not be permissible for you to marry her younger sister until the older sister's `Iddah ends. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.