mufti sb,slam i am hanfi sunni and wana ask a question that if a man does sex with her real blood sister then does nikah of there parents become invalid/haram.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If a brother commits incest with his sister then this does not have any effect on the marital bond of their parents; rather, the bond of marriage still exists between them.

However, committing Zina (fornication or adultery) with a Mahram (a person whom one is permanently prohibited from marrying due to suckling, marital or blood relations) is absolutely the most heinous and abominable form of Zina as stated by the scholars . For more benefit, please refer to Fataawa 89280 and 82317.

Allaah Knows best.