Fatwa ID: 02309
Answered by Maulana Maruf Ahmed
Assalamu alaikum, If a brother and sister have sex or do sex related things with each other before their marriages does it affect their marriage to their husband and wife and does it affect their parents marriage. Also does the sex or sex related things break the blood relationship of brother and sister or not and does it have any effect on their relationships with each other's children as in nephews and nieces. Thanks
The form of incest that you mentioned, although it is an extremely despicable sin, does not affect the validity of the marriage of either the brother or sister. Likewise, blood relations between the two will remain. 
And only Allah knows best.
Written by Maulana Maruf Ahmed
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham