A family home would not be considered private rented accommodation for the purposes of rent allowance.
If someone has accommodation, say a house or apartment to rent, it is possible to rent it to a family member, if the landlord can prove that they were previously in receipt of rental income from that property. In other words, the property had been let and is now being re-let to a relative, which proves that it wasn't bought for the purpose of providing accommodation to the relative. A room in a family home would not fit this criteria.