Condoning a haraam

A Muslim family adopted a female child after birth. Her mother was a Christian. They brought her up as Muslim. Now she is 30 years old and wants to marry a Sikh boy. Adopted parents are in a dilemma trying to persuade her but she is adamant to Mary Sikh boy. They told her they have nothing to do with her. Adopted parents want to know can they go with her to the Registrar's Office and stand as witness and later bring her home with the Sikh boy and members of her immediate family and offer them food then she goes away not to return in future. Is this acceptable or is it haram? Please advise.