I bet those examples result in more deaths than the hottub you've pictured.I still believe we live in a free society where people can do what they want with their property and grown adults can choose to participate and take personal responsibility for the outcome. 

In some areas they might even try him as an adult (do they still do that to kids???).

I've been renting to the same family (elderly couple and their adult daughter) for the last 3 years with no increase.

Like a job that get people’s movie tickets, it looks fun, it feel like handing a adult a candy, they seems excited lol

Check the references for EVERY adult.  

I have witnessed cases where parents ultimately destroyed their adult kids' life by letting them be irresponsible and living obligation-free in the house.

How schools provide such adult education differs, but they all provide something along these lines.

Just tell her for liability reasons you need all adults on the lease and need to screen her sister the same as any other adult.

Exclusions that may be in a policy would be discrimination, sexual harassment, Intentional/criminal acts on your part. 

Please familiarize yourself with the facts before you start lecturing.From the TSA's website:"Effective June 21, 2008, adult passengers (18 and over) are required to show a U.S. federal or state-issued photo ID that contains the following: name, date of birth, gender, expiration date and a tamper-resistant feature in order to be allowed to go through the checkpoint and onto their flight.