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I am afraid under PA law, the landlord is not generally the insurer of the safety of the tenants. That is the general premise that the courts start off with. However, a lease, even a month to month lease, gives you the right to peaceable use and enjoyment of your premises. If this use and enjoyment is being interfered with by the neighbor's violations of the terms of the lease (by having a person not on the lease living there), then you need to start by giving written notice to the landlord that he is in breach of the lease and as such you would seek to sue for breach and all costs of moving if he fails to cure the breach and you have to terminate your month to month lease based on that ground.
The problem is that as a month to month tenant he can terminate your tenancy with 30 day notice for no reason at all, so if you complain in writing that this is a breach of your lease guaranteeing peaceable use and enjoyment of the premises, he can give you an eviction/termination notice and then you have to sue for retaliatory eviction based on him evicting you for exercising your rights as a tenant.
The other option is that if the son is creating a nuisance, you could sue the other tenant for the nuisance and seek to get a restraining order forbidding the son, who is not on the lease, from being on the premises.
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