Illegal Tenant - how to evict?

My head is wrecked, looking for some advice as to what I can do.

Have a 2 bedroom Granny Flat, it’s part of my primary residence. As in when the house was built a section of it was specifically designed to be a Granny Flat. All legit, planning permission etc. I bought the house this way and rented it out the flat under the rent a room scheme. From Google research at the time this is correct as it’s part of the main house so qualifies with revenue.

Had two lodgers, all going good until one of them allowed their sister to move in as a “guest” initially to sleep on the couch. In September, Without my permission. After two months of this I challenged him as my home insurance only allows two extra people. After back and forth I said she could stay until mid December but had to be gone by this date. She was not paying any rent, was using my wife and bins and basically costing me money. I expressly told him she could not move in permanently.

In the meantime he engineered a situation and made it so uncomfortable that he forced the other tenant out. Again I expressly told him not to do this but he carried on regardless

It’s now mid January, he and the sister are there, other tenant is gone and he is paying the rent supposedly on his own. I still do not want to her in my premises, she is an awful individual I won’t go into it….

She has no lease/rental agreement and has never received my permission to move in.

I gave the original two tenants a 6 month rental agreement/room rental lease to sign in November (would have ended in may) but it wasn’t signed by either at the time because of the hassle. I have asked the remaining tenant to sign it but he never did. So there is no signed lease in place right now.

Met the brother & sister last night and told them I want them both to move out by end of February. They laughed in my face and refused. Said they will talk to a solicitor and that the Granny flat is a separate building and I can’t make them leave. She is his guest and can stay. It’s been 6 months, she is not a guest, she is a lodger at this stage.

What ever about the guy, I want the sister out now. How do I go about this legally? Surely she cannot move into my property like this and just stay? Tell me the law is on my side here!