Subleasing...Add them to the lease?

[Kevin Howard]

I recently had two girls move into one of my 3 bedroom apartments. One of them was suppose to have her 17 y/o brother (she's the legal guardian) move into the third bedroom to assist with rent. Only the two ladies signed the lease since the brother was a minor. They've been there for two months and the brother never moved in so now they're asking if one of their friends can move into the 3rd bedroom to assist with the rent. I've run a credit/background check on the potential tenant and have no issues with her moving in. My questions is, would you add her to the existing lease via an amendment of some sort or would you allow them to sublease and not mess with the current lease?