Wow! I wonder if the tenants that you have been paying the rent to have the authority to be "sub-leasing" the guest house. Do you have a written lease agreement with them? If so, check your lease to see what it says if the property is sold. Lease agreements in writing and signed by all parties must be honored but only if the tenant (landlord) has the authority to be sub-leasing the property.
And, it is best to consult a real estate attorney in this matter as it is a legal one so is best answered by them.
Good luck to you!
Prudential Connecticut Realty