If a man have sex with his step daughter does that invalidate his marriage to her mother ?
This person is a wicked person. If he is under a Muslim ruler, he should be punished with the Islamic punishment for fornication (which is stoning to death if he is ‘Muhsan’ (if he ever had sexual intercourse with any of his legal wives)).
It is better that his wife doesn’t stay with this wicked person and she should seek Khula (woman seeking separation from husband for legislated reason). And the step daughter should be veiled from this person forever.
The fornication is not a legal marriage so this doesn’t effect the marriage with her mother and there is no proof that the marriage is invalid.
Answer by : Shaykh Abu Bilal Al Hadhrami
Translated the summary of answer by : AbdunNoor Al Hindi