Tenant wants to add her 15 year old sister lease in MA?

[Lisa Hoyt]

So...I help "guide" my sister with issues pertaining to her rental property in MA.  Which means I am the middle man so she doesn't have to deliver unfavorable messages since she is an owner occupant.  She can "blame" things on "the property manager".  It works very well - usually.  Now, however, they have discovered that the 15 year old sister of one of the two adults named in the lease, has been staying there.  When I asked about her (we thought she was over 18), because male tenant said "we are thinking of having her stay with us for the summer" (violation of lease which says visitors can stay 7 days), the female tenant replied "he misled you, sister visits for help with homework, she spends the night sometimes".  Eight hours later, female tenant emailed again saying: "Sorry, I was at work earlier, here is the whole story.  Their mom died 7 years ago, sister was being raised by an aunt who recently remarried and no longer wants the girl.  She is a good girl in a bad situation.  Partner and I would like to have her live with us, could we change the lease, pay more money?"

They re-signed a year lease on May 31.  This conversation started the VERY next day, June 1.  My take: nice couple, helping teen sister, respectful of property, have been late with rent a few times, but not recently and not more than 2-3 days.  My SISTER's take:  they lied! They are manipulating me into having a teenager under my roof.  THEY LIED! etc.

I am not a lawyer, just trying to apply common sense with what I find out about MA/Federal housing laws via google, and as I understand it, if they get custody of the girl she becomes a part of their "family" and families are a protected class, therefore she has no recourse, but to let her stay there.  Am I wrong?

I have also suggested she: 

• consult an attorney who deals with MA real estate, 
• move and stop being a landlord (which they were considering for health reasons- selling this house would allow a cash purchase of a new home), 
• move and continue being a landlord, renting both units hoping that the financial gain will help her better deal with tenants who are less than truthful

I would be most grateful for thoughts/input/ideas, particularly from people familiar with the MA laws which are very owner unfriendly, or so it seems...Thanks!!