current tenant wants to add a person and dog on lease

[Marysue Connelly]

We have current tenants (a couple w/ 2 young kids) that have been with us for 4 yrs.  They are good tenants. This is a single family home 3 bed 3 bath w/ a lg basement. It is currently rented for under market value by about $300-400. We pay water.  Her sister wants to move in with her dog.  The property is no pets.  I'm open to having the dog w/ a pet deposit and pet rent.  We are having her run a background check now.  I would like to chg $500 pet deposit (refundable) and an extra $100 a month total for the sister and her dog.  What would you do?  Is this enough? too much?