This is the update and end of the thread. Yesterday, on The BP blog was a post on "How to Raise the Rent" which gave me enough new key search word ideas to find this from a MA.GOV pamphlet called "Landlord Rights and Responsibilities":
If you have a written lease and do not have a housing subsidy, your landlord cannot increase your rent during the period of your lease, even if you and your landlord agree to an increase and put this agreement in writing. For example, if you signed a one‑year lease that runs from September 1, 2008 to August 31, 2009, your landlord cannot raise your rent in January 2009.
The only exception to this is if the property tax has risen during the year, the rent may correspondingly increase. Thanks for helping me sift through the finer points of this issue, and maybe this case study will help landlords in other states with a similar issue.
My final determination:
The teen sister moving in can have no legal bearing on the rent required until the lease is up for renewal AND it is familial discrimination to refuse to allow the sister (assuming legal custodian) to live with her family.
Thanks for your help.