Jonathan, I am not asking for legal advise, I said advise, without the word legal. 

“Broker” is a legal term that often requires standardized coursework and a state license. 

Is there any legal action one can take or is it just a lost cause?

I won't be able to make the cottage legal rental without living in the primary home myself.

This is something their screening probably would have caught, or their system would have a legal procedure to deal with it.

Hi @Heather Bailey something to consider with this is legal mother/daughter may not mean legal duplex. 

Signing the lease demonstrates they agree to the terms and then you can hold them accountable.If you don't do this, then the person named on the lease and paying the rent could disappear and you would have no legal authority to charge the remaining three for unpaid rent.

What happens during periods when the demand for legal work is lower?

Just to add to what has been said here - if the seller described this as a triplex on the sale documents, they might have a legal liability.  

But I’d’ also ask if the girlfriend is a legal US citizen?