Hello and thank you for your question. I will be very pleased to assist you. I'm a practicing lawyer in England with over 10 years experience.
Are you seeking to establish an LPA for yourself or a relative please?
Thanks. The legal test is laid down in the mental capacity act. She must be able to listen to or read information about what an LPA is and hold this in her mind for long enough to weigh it accordingly and to be able to make a reasoned decision upon it and demonstrate that she has done so.
This is a very simple process for you and I but for some people, e.g those with advanced dementia or even more obviously those for example in comas and so on it is either more difficult or in deed obviously impossible.
You will need someone to certify that in their opinion she has capacity (this cannot be the attorney). That person must either be someone who has known her for more than 2 years or a professional such as a doctor or lawyer. If your sister has any mental difficulty of any kind it can be wise to ask a GP or consultant to provide a certificate and/or provide a medical report in case anyone challenges her capacity to make an LPA in the future,.
Is there anything above I can clarify for you?