Legal Question in Landlord & Tenant Law in Pennsylvania

So, if I understand correctly, there has always been the same number of people. There is, I assume, nothing in your lease about the maximum number of people allowed. And, I assume, you are in the middle of a lease period. All of that being said, the landlord cannot require you to vacate as a result of this. Also, a notice to vacate by mail is invalid. A "notice to quit," which is what it's called, must be personally delivered or placed on your door. Should the landlord file an eviction with the magistrate, it should be dismissed on the basis of a lack of proper "notice to quit" before you even get to the merits of the case.

Of course, at the time of lease renewal, the landlord may choose not to renew the lease for any reason (as long as that reason doesn't violate the law). So, he may, at the time of lease renewal, say that he is not renewing the lease. However, in Pennsylvania it is illegal to not rent to someone due to "familial status," which means having children. Unless the premises is unsafe for the number of people living there, I would think that you may be able to challenge that as well.

(I must say, though, that I am only basing this answer on the information which you gave. I have no idea the size of this apartment, for example, or whether the landlord may have another valid reason for not renewing the lease.)