OMG, being landlord is so hard

You are not only adding the fiancee to the lease, you are also requiring that both/any of them responsible for the full rent, following rules and regulations, dealing with division of security deposit and responsible for certain repairs.  It is a complicated agreement.  It is not just "whatever she does, I do".   They could break up and leave you with a mess of who is in charge of what. 

A new lease is recommended. 

http://www.nolo.com/legal-encyclopedia/adding-roommate-lease-or-rental-30187.html