Not sure Motability will see it like that mostly because of Daily Mail headline stories saying things along the lines of “non disabled drivers driving around in Rolls Royce’s at taxpayers expense”.
Load of tripe of course but you see why Motability have to make sure the rules are followed.
I would go along with JS’s post and have Simon check out the exact procedure with RSA and Motability and if they won’t ware it get a refund of his deposit and as there are no rules on what his sister does with her PIP money Simon could look into doing a private lease or buy on behalf of his sister with her PIP money.