Both sides have female tenants.

My best tenants are single females Age 25-35 . 

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. 

And, while that might have been worth discussing had they approached you as "adults," they chose to financially harm you. 

I'm planning on making an amendment to her lease with her "guest" on it stating liability and an increase of $30 per month to cover the utilities I cover for another adult.

They are a two adults, two children family.  

I also make the tenant identify who will be occupying the property and the lease states that only the identified folks can stay on the property (all adults get screened, even if they aren't signing the lease).  

You have a contract with two adults.  

Some states require landlords to look at the total income of the tenants, like CA, but others allow landlords to require that each adult can pay the entire rent on their own (3X the rent, etc.).So, it just depends.  

I have 5 adults (and 2 dogs) wanting to rent one of our brand new houses.