Tenant's in-laws overstay

[Robert Modero]

My wife and I are landlords and live in the first floor of a two family dwelling. Even though the house rules state that a tenant is allowed to have guests stay with them for "up to a period of two weeks", over the summer our tenant asked permission for his in-laws (who were coming from Europe) to stay with them for a month. My wife consented, and at this juncture it appears they've been staying here closer to six weeks... and thus far haven't given us any indication as far as when they intend to leave?          With their in-laws living upstairs, what started as a family of four (the husband, wife, and two small children) has expanded to six people living among four rooms, with a corresponding increase to our water bill - having a washer and dryer at their disposal.    We just renewed their lease in August, with what now seems like a mere pittance (.06%).                                                           My question(s) is: Would I legally be within my rights as a landlord in NJ to charge the tenant extra $$ for every additional week his in-laws continue to live here, or do I just ask him point blank to state his intentions - as far as how long they intend to continue to live here, giving him an ultimatum for them (his in-laws) to leave, preferably sometime before the end of this month if his response is nebulous? Any advice or info from handling related experiences would be much appreciated.