Full story below: Skip to the bullet points for the tl;dr.

I purchased a few properties to be a rental properties from another landlord, who is something of a slum lord and was willing to offload some houses that need work for a great price with seller financing.

There was a miscommunication between him and his property manager, and the manager put a new tenant in one of the properties about a month before closing (shortly after we signed the paperwork). The tenant is month to month. I am in Washington state, and not in one of the big urban centers like Seattle that have unique tenant laws.

He was apologetic about it but the rent was high enough to cover the payments so it didn't kill the deal- I just figured I'd give them notice down the road when it was convenient for me to make improvements to raise the value of the property (and if they were great tenants, maybe leave them in for a while).

Well, we closed early last month and the seller gave me the pro-rated rent. I decided to make a few minor safety repairs off the bat (including carbon monoxide detectors and smoke alarms- I'm not kidding when I say he was a slum lord!). The tenants are two sisters who cosigned. The the first tenant, who I will call "A", was professional, composed, and gave me a good impression. Her sister, who I will call "B", was tattoo'd up and looked a bit less professional and didn't talk a lot, but left me with a neutral impression. They had children in the house of varying ages (one late teen, one likely around 10-12, etc). I'm not sure whose kids are whose. One of the times I stopped by there was a an adult male in a bed in the living room sleeping.

When my contractor went to do some fixes, he mentioned to me that he found their living arrangements strange; he had to put up a smoke detector while the older teen had a man in the bed, for example.

Over this weekend (approaching the first month I would be collecting their rent myself), Tenant "A" texts me, and then we have a phone conversation. She relays the following information:

•  "A" makes just enough income to cover the house herself. "B" does not. "A" is also going to college currently to go in to a better career.
•  Her sister, "B", decided to allow a man/boyfriend that she had only met three weeks ago to live with them. He pays no rent.
•  "B" has brought drugs and guns in to the home.
•  They have had multiple disputes about what room "A" is to sleep in because of this.
•  The other night, she twice lost her homework due to power flickers because B's boyfriend was messing with the electric panel so he could install hardware to grow weed in the closet.

As a result, "A" is giving an ultimatum- either boyfriend leaves, or she does. She asked if I could take her off the lease if she moved, and I, of course, told her that I could not. She also wanted to know if I, as landlord, can tell her sister that her boyfriend has to go.

I obviously don't want this guy in the house. However, I'm a little worried; my mother, a few years ago, gave a similar ultimatum to a renter who was growing weed, and he trashed the house in response. (Luckily, this one needs renovations.)

I also don't know my rights as landlord. Do I have the right to tell tenants who they can/cannot have as guests? Can I threaten eviction (ending of the month-to-month lease) if he does not leave?

tl;dr: bought a house with a newly-placed month to month renter in it. One of the renters tells me the other renter is letting a drug dealer live in the home who is modifying the wiring and wants me to kick the drug dealer out. Can I make this specific demand under threat of eviction/termination of month-to-month lease?