Namely, that if a tenant (in this case accompanied by a whole family) ends up stop paying rent and I would be on the hook for both covering the mortgage and dealing with open ended legal costs and time.

We use RentRedi for tenant screening and management, lease documents, automated late fees, messaging, maintenance requests. 

But the real kiss of death was their arrival and documentation (with photos) of the poor & inconsistent cleaning.

Hi @Heather Bailey something to consider with this is legal mother/daughter may not mean legal duplex. 

Any ploy, tricks and legalities.

If you are a legal property manager in your state the owner should have the unfettered ability to allow you to act as his/her agent to enforce the lease, whatever provisions they may contain plus whatever your state laws allow.

Registration is only permitted for legal entities, partnerships and natural persons who are 18 years or older.

Being in NY maybe something to do with physical document transportation?

The increased risk is generated by the fact that the owners are relying on the validity of the tenancy in common Agreement for their usage rights, and it is possible to imagine legal circumstances under which this validity might be undermined.- If the subdivider or someone with relationship to subdivider sells, a public report must be done.

I am sure a Thumbtack PM works great when something legal goes down lol.