A rental is just a means to an end for renters; a garment they wear, then discard when it no longer suits them. To them, landlords aren't people. Also, all landlords are rich
and that means landlords can afford to
share!
People use various dodges to get away with relatives, lovers, subletters, and pets who aren't on the lease. We're all familiar with the service-dog dodge.
For people, there's the "he's/she's my babysitter" dodge, the "just letting friends stay while they find a place" dodge, and the indignant, "But she's family! She had no place to go! Any decent person, etc. etc." dodge.
You have to be willing to throw them all out and re-rent, if one won't sign a lease. If you are the least bit worried about re-renting, or about being sued for something, they've got you where they want you.