Should I act as an informal managing agent

Other

I own and manage 3 rental properties and have made it my business to learn the legal and practical requirements for letting property in order to be a 'good landlord'. My Brother-in-law is about to buy his first rental property and has asked me if I will manage it for him, for which he will pay me a fee. It wouldn't be worth my while doing it formally, (registering, getting insurance etc) for one property but am I ok to take it on informally? What are the risks to him and to me? How should I describe myself to his tenants?