Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first.
If your father has full mental capacity why do you fear that his wishes will not be followed?
The fact that you live so far away does not preclude you from being a Joint Attorney with your sister - it simply makes it a little impractical if you BOTH have to sign things. However it is possible for attirneys to have "joint and several" responsibility so that either of you could sign and it is actually far more sensible to have two people named in case of unexpected illness or the death of one of them.
The fact that the Power has to be Registered immediately is not a problem - your father can over rule any decisions made so long as he has mental capacity
I hope that this is of assistance - please ask if you need further details
I am afraid that there is very little that you can do other than the options have outlined above.
If your sister does try to make decisions for him whilst he is mentally capable then he can simply revoke the Power of Attorney