Inheriting a tenant from RE deal

[Serah E.]

Hey guys,  I am close to closing on a full duplex that I am currently renting from my landlord.  In the contract he asked if I could let the tenant on the other side stay 6 months after the closing date.  I agreed to this.  He doesn't have a written lease for the tenant, I guess she is on a verbal month to month basis.  When this agreement occurred, she was living alone (she kicked her sister out due to drug use.)  Now her sister is back at home.  Because of this, my partner thinks we should charge the sister for living there or evict the tenant for breach of lease, since the purchase agreement states the term "tenant" as oppose to "tenants".  Plus, we have our suspicions that the sister is still on drugs.  But the problem is...there isn't a lease agreement so how can it be breach??  All constructive advice is welcomed.  Thanks.