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