Helping family...

[JamSam17]

If my brother comes to stay with my girlfriend and me until he sorts his own situation out, who do I have to inform? The council, etc?

[Ethel]

If you own your own home - your insurance companies.

If you are renting - your landlord and insurance companies.

If you are on benefits - the relevant agencies that handle them.

[JamSam17]

Thank you so much Ethel, we do own our own home, so I'll get onto that tomorrow.
What about council tax, maybe?

[bednobs]

you wont have to pay any more council tax. Your brother will have to inform them that he no longer lives at his old address though or he'll have t continue paying

[QuercusRobur]

Never tell any-one any-thing. It only confuses them. They'd much rather be left alone. I speak from experience, and hope it doesn't sound facetious.