They must be separated immediately. They can’t be married and their marriage is void.

My friend Alhamdulillah recently reverted to Islam.When he was a hindu as per the tradition he married his sister’s daughter. Now his question is since he has embraced islam, is his marriage valid as Islam doesn’t allow marrying sister’s daughter?

Invalid Marriages | March 14, 2015 2:14 pm
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I want to know if it is halal to marry a divorce women with three childern which they cant tell her parrents due to a social reason which is that the man she is getting married to is not meeting her status in socity which they decided to marry in secret until the person becomes rich. they marry with two witnesess in secret. now they have this relation since 4 years and they still meeting each other in secret is it halal in islaam.
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I am a previously divorced mother of one child. Last week I had my nikah done. I originally asked my brother to be my wali and support me in the nikah but he refused. Therefore I appointed the imaam of the mosque to act as my wali. The nikah was then done in mosque with the presence of two witnesses and haq mehr (money) was paid to me. Is this marriage valid?