If he is serious, he can try to take you to court.

I’m not an attorney but I would think if this “new zoning” affects the resale value of your home (rental or otherwise) you have a pretty good case in court. 

Can you get a court order to sale (partition) if you cannot locate the heirs of an estate?

Repairs, improvements, legal/court fees, travel, etc.You don't "write off" the mortgage itself, but you can on the interest.

You might add the "shoe leather" avenues:- going to the county superior court and collecting:- divorces- evictions, get the landlord contact (plaintif)- delinquent real estate taxesDoor knocking FSBO signs, calling FSBOs in craigslist, using peoplesmart.com searches on owners of tall grass houses...

I believe there are a few states where there's a court case that says the MLS can't be Realtor exclusive (# 6 http://www.realtor.org/law-and-ethics/mls-access-litigation-summary), but in most states you need to have a license or be an assistant to an agent.  

A lot of the housing near universities are not operating in compliance with zoning; see recent court ruling here:http://planphilly.com/articles/2015/10/13/landlord...

Centre Pointe ***’n decision is now out: The Supreme Court invalidated that association’s rental cap amendment but did not invalidate all rental cap amendments.

Its easy to look up in court records all the suits involving tax sales. 

If you've covered your bases with the water and sewer, the sister then becomes nothing more than a houseguest if you end up in a bad situation, with no legal rights other than what she might establish in court.