Do you have more lines than just yours on your account? If so, go to WWW.att.com/transferservice and authorised your mom to take over all lines on your account except yours. Then whomever owns the account that your fiance's number is on needs to go to the same website and authorised you to take over her number. Then you have an account with hours and her number. A few things to remember, if any of the lines that are moving have an unlimited smart phone data plan, they will lose it. Also all numbers on a family plan need to be in the same area, so if your number is in one area (example Texas) and her number is in another area (example Oklahoma) then in order for you to be on the same account and a family plan, one of you would have to change your numbers.
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.