Does your fiance want to have the birth mother's parental rights revoked?
Are there existing court orders regarding custody, visitation, etcetera?
I think the best way for you to proceed is to seek legal advice. You and your fiance should make an appointment with an experienced Family Law attorney.
Write down all of your questions and take that list with you to the appointment along with any court documents your fiance may have (especially the most current Order).
The attorney should be able to advise you as to how best to proceed.