I would suggest you get medical evidence to confirm his mental capacity regarding making a will etc.
You will need this.
If your father is able to understand he does not need permission of his guardians to do anything.
He has the right to deal with his affairs. If he lacks the capacity then he cannot make any decisions and what he does do is void.
The people appointed as guardians have the power to do what he can and to look after him. It does not mean he loses all rights to make decisions is he has the capacity to do so.
If your sister is going to try and have herself appointed, they have to be notified because they have to be given the revocation of their appointment. If it is through the court they have to neb served with a copy of the application and will have to appear.
I hope this makes sense and is of assistance., if there is nothing further
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