How can she tell her non-Muslim mother that her husband is going to take a second wife?

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Check below answers in case you are looking for other related questions:
Is it a sin if one does not feel comfortable with a particular person?
He is homosexual and is afraid to get married

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She has committed sin and wants to repent
Her husband has changed completely and treats her differently, and he commits haraam deeds; what should she do?
Her father is forcing her to attend gatherings in which there are innovations (bid’ah). How should she behave with him?
Is it a sin if one does not feel comfortable with a particular person?
He is homosexual and is afraid to get married

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