A person who is a family member can claim HB under certain circumstances, I think.
It has to be a proper tenancy, the deposit has to be protected, and you have to be charging the market rent. So you're a "proper" landlord. If you're in Wales, you'll also need to register under the Rent Act 2016.
Bear in mind as well that the council will cap the HB according to how many bedrooms are needed (it's called the LHA).
Also, if your relative is already claiming benefits and qualifies for HB at the moment, a new claim for HB may trigger a move to Universal Credit.
Check the Shelter website.