They could also have rules on how many unrelated adults can live together in a unit. 

Generally people here tend to prefer houses to condos and also I've noticed a big drop off in demand on places renting for more than $1400/month..unless it's a big 4/2 or 5/3 kind of thing that the college kids or young adults will go in together and share.

You are not their parents or a baby sitter.Tell the tenant that she can leave when ever she chooses and when she does inform the remaining tenants that they will still be responsible for the entire rent.Do not get involved any farther than that, it's not your business...they are all adults.

I’ve also lived in apartments my entire adult life and don’t want my first property to be a live in duplex or something.

How can I place a provision in her contract that stipulates if an adult moves into the unit, not listed on the rental agreement, I will automatically increase her rent?

I tried to sue all 3 of the adults in the house, but I was only able to go after the 1 person who actually signed the lease, although all 3 of them are listed as tenants and the other 2 are listed as occupants as well.

Nothing like getting free or discounted tickets to events through roommates who are generally happy to live with you and are responsible adults who need a safe, quiet place to retire after a long day on Main Street.

But I only allow an additional child not an adult.

Secondarily, I’d like to be in a position to be able to give my daughters something of value (property)as they start their adult lives.  

I want to be able to give young adults the prospect of another path that they can go on as adults.