I came across on youtube an OST video with all dialogues clear and in one of them hareem says on phone to seher that " maine qasam khayi thi nikah ke paper sign karte waqt ke mai tumhara naam apne shohar ke naam se khurach doongi shadi ke baad". So it seems seher was aware at the time of her nikah that zamin and seher are married.
https://www.youtube.com/watch?v=2efKxhDMdG4 ost with all dialogues
well whatever it is, zameen´s character shouldnt be ruined in this second marriage circus