You have an uphill battle gettinv custody if the girl has been with her dad this whole time. Also you marrying a guy with a sex abuse conviction, especially if he was a register sex offender, is poor judgement and DSS would have removed the child from your care most likely. Almost no chance of succeeding.
You'll have to call around for an attorney. Look in the yellow pages. ?don't count on any attorney taking this case pro bono.