Most adults don't like being told by other adults how to live their lives, manage their finances, drive their car, raise their kids, etc. 

I ask; When are you looking to move, how many people would live there, do you have any pets, do you meet the criteria of 600 credit score, 3 times income to rent, no felony convictions, and no evictions, and are you willing to pay a $40 application fee for all adults to check that list? 

That would be a hard pass, for me.Set your criteria - all adults 18+ must have credit and background checks done.

They are 3 adults, early 60's, employed, 2 sisters and one sister's husband who looks like ZZ Top ... the house is actually too small for growing weed inside, but has a huge yard but totally exposed on a corner lot, so i dont think that would work.

Tears, sob stories, the whole it's not my fault cry fest and the like all have no place here in a business forum among adults. 

Am I legally allowed to turn down a tenant that would be living there with another adult and 2 kids for example?

Tenants are adults, not children. 

@Ted Klein combine income and put all adults 18 and older on the lease jointly and severally.

There's no way I'm taking that person beer to schmooze them into behaving like an adult. 

I wouldn’t over-complicate a roommate situation and typically if they are normal human beings, they will be ok sharing responsibilities for shared items like cleaning products, toilet paper, etc and you can split those costs like adults.