The only issue I can see is that the buyer’s mortgage company and solicitor will demand ‘vacant possession’; ie normally meaning there are no tenants in the property at completion. Lodgers aren’t the same thing, but it might present an issue? I’m sure not insurmountable.

Obviously, one for both solicitors to be asked. Might end up with something like asking the lodger to move out for a night on completion day, before the seller, and then for the buyer to move in before inviting the lodger back?! That would enable everyone (solicitors included) to sign the paperwork 100% honestly?

The buyer may have difficulties getting finance and lenders want Vacant Possession of a property when it comes to a residential home. You will be asked to declare that it won't be vacant to your conveyancers who will have to inform the lender.

So have your sibling start talking to a good mortgage adviser today, to work out a lender that will be content with the arrangements, or, organise a temporary "eviction" of the lodger to only to re-sign a new lodger agreement with new owner.

The lodger just signs a form to say they don’t have any claim on the house. Ask your solicitor.

Slightly different but I bought the house I was renting and kept my roommate on as a lodger. I went to a mortgage broker and he said some lenders wouldn't lend under the condition but I still got a good offer from a major high street building society. My (now) lodger did have to sign paperwork stating they were aware they had no rights to the property. My recommendation would be don't try and hide it, declare it on the paperwork needed as it keeps everything above board should things go wrong later.