Tenant wants niece to move in

[Tony Murphy]

We've had a fairly good tenant (single mom with 14 yr old) renting out an apt in our duplex for the last two years.  I got a call that she wants her niece (18yr old) from Texas to move in short term for a few months until she gets her feet on the ground.  She gave me the whole story i.e. moved here to be with a boyfriend, broke up, she's a great kid, etc.  My tenant pays fair market rent already (in fact a little more since rents have gone down slightly in the area) so I'm wondering if I should still ask for additional money each month to at least cover the increased cost (water/sewer, etc).  I also plan to have her niece go through my screening process and fill out her own lease.  Am I missing anything else?