Tenants wants guest to stay for 3 months

[Dimitri Baldwin]

Hi. I own a multifamily that I currently live in and rent the other space to another family. We rent to another couple with their 2 kids. Our tenants have been with us for 3 yrs and consistently pays on time. However, our tenants want to have their family members take turns and stay with them for 3 months at a time. The guests that will be staying are residents of another country. The lease says guests are allowed to stay for a max of 2 weeks. We are willing to accommodate their requests(for now) since they have been really good tenants for us. What is the best way to go about this so that we can still protect ourselves legally as landlords? Especially if this happens again in the future.  We live in NJ. 

Thanks again for all your help!!