I think the biggest issue is they all start with a "Cash invested" amount and I have no idea what mine is at this point so I am lost out the gate.  

I’m concerned that it’s a fire or safety issue? 

We live in 1 unit and will rent the others.The issue at hand is that one tenant has/had a verbal contract with the previous landlord (they are family). 

This is a complex issue and I know the community would benefit from learning about our decisions.

What's the legality of doing the same here?

My question is can I legally keep their security deposit?

Small slivers of land, lots too small to build on, no water, no power, landlocked, flood zone, dump sites, no sewer, but too small to add septic, around where I live counties require a certain amount of road frontage so people won't put mobile homes, so you can buy OTC without the required road frontage and lot could be more or less worthless....can't build on it,  and all other kinds of issues. 

Are there any legal ways of going about this?

The squatters would have to go through the legal process, which they won't because they know they have no evidence of legal occupation.The worst that could happen is violence, so you should only attempt this if you are prepared to defend yourself. 

They can also report maintenance issues and see their payment summaries as well.The app has a slew of other features - complete book-keeping, automatically applies late fees, can track recurring charges, etc.