As always, it is up to you!
Check your contract with the tenant(s):
- will overcrowding occur, if so, say “No” to brother. 3 bed implies no over-crowding
- this is subletting, if not temporary, so inform the tenants that they take on the full practical and legal responsibilities of a landlord, that in no way changes their responsibilities to you: see a similar situation at Tenant wanting to let a room
If not renting a room, they can stay with your written permission. You may need to undertake a right to rent check to cover your back.
Regarding the suspected boyfriend, I have the same situation, but cannot prove it, and my tenant is unlikely to admit to it. You should be able to pick this up via clues during a routine inspection, and may be fobbed-off by “only a visitor”. It could be the boyfriend, or the brother! Nothing you can do until you get proof and even then it must contravene a contract clause and you have to decide to turn a blind eye or lose your otherwise good tenants.
If the brother, the tenant is trying to regularise the situation, but not saying so: he may have stayed longer than expected. The same applies to a boyfriend.