Tenant's sister stays a week each month for business - problem?

[Janice R.]

I just found out through my PM's new monthly statements that the tenant's sister stays with her one week each month - and she apparently pays the whole rent.  The tenant has been there for almost 3 years and the rent is always paid.  When I found out about the sister, my PM said that the sister is "very nice" and didn't think that there was a problem.  However, I worry about any liability or any rights the sister has now on the property.   

The tenant's lease states the people who live in the property and "no others".  Does anyone have any thoughts about this - this is in Arizona.

Thanks.