Hello, my name is ***** ***** I will do my best to assist you.
Effectively, he would need to end his LPA with his sister, and make a new one.
Here is the information about ending this:
And the information about making a new one:
It costs £83 to register a new LPA.
But it may be possible to be exempt from the fee - see here:
Is there anything I can clarify for you?