I cannot marry my wife's sister, what should I do now?

Anonymous
I am a Muslim and we can marry upto four women. I have a very good friend and she has a very beautiful younger sister. I want to marry both, So I proposed my friend, she and her parents accepted and we will get married soon. I planned to marry her sister after a couple of years, but booommmmm... There's a problem, in our religion, we can marry two women but not two sisters. (I didn't know about it)
What should I do now?
I cannot marry my wife's sister, what should I do now?
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