He called me last night to let me know what had happened and that he had filed a police report but couldn't get a hold of the mother because she was working.The mother works all hours and the boys are being looked after by their teenage sister.My question is: Is any of this my responsibility? 

More general question is HOW do I accurately find out what the cap rate for an area/property should be?

@Jeff Filali Thank you for taking the time to write up such a long response!

Just a couple quick responses (and I'm no expert so you should probably verify with one if necessary):If he doesn't pay and the building is collateral then you would have the right to foreclose, however, foreclosure in NJ can take a long time and be costly.

I got it back and they left a large portion of it blank with the explanation "I'm still going to be responsible for the full lease, so Abbi didn't fill out the income part, and her lease history is the same as mine."

@Brian Eastman thanks for the response.

No attorney can really help you as the bank has no responsibility, or perhaps even the legal ability, to negotiate anything with you.You have no recourse against the county or the courts.  

@Matthew Auletta Hi Matt, thanks for the response.

@Lisa Ramos if your numbers are accurate, then investing the money to bring the house in line with the current market makes sense. 

Also recognize there is an inherent problem with getting accurate numbers on small properties.