Don't want to go into too many details as RL people know my posting name.
Surely though, if person A owns a property, and person B lives in it, they should pay person A rent which would cover the mortgage as well as paying for the bills?!
Person B wants to just 'pay for what they use' and thinks that person A should still pay for mortgage as they are family and person A owns the property and it wouldn't be an 'official' renting situation.