Infact SEHAN were drunk, while they were discussing abt their past life... But before and after that Rehan had said many things abt his Ammi to Seher.. So by seeing Rehan at Ibrahim mansion, and his connection to the family.. Seher, being an intelligent girl as shown would easily decode the situation( i think so... cant trust cv's anyway)... i think she will be the first one to find out Billi-Rehan relation!!