Problem Tenants

[Jo Ei]

I have tenants in building that have been fine to deal with until now. Now in the summer months, they are up till all hours drinking and swearing in the yard. They pay early and have not caused any other problems to this point. They appear to be having some domestic issue. The brother of one tenant moved in. He clearly has a drinking problem. I see him drunk in the morning, afternoon, evening. He is always polite when I encounter him but he is the source of the friction at the property. The late nights can not continue, it is disturbing others. The original tenants are a couple. They are month to month tenants.
So, approach the orginal tenants and tell the he has go, or tell them they all have to go? I know domestic issues can be sticky. So I want to have the right approach from the start. Any ideas?