Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
Has your mother ever signed a Lasting Power of Attorney?
Is that for Finances or for Health and Welfare?
You should have been given notice - when was it signed?
Do you remember seeing details about it at that time?
The Power of Attorney deals with Financial matters only.
It does NOT mean that his sister has the sole right to make decisions about the care that your mother -in-law receives.
In terms of who decides it is in the end up to the doctor to say what is best for the patient - and your sister in law cannot go against that - and if she tries to then the majority of the siblings can over rule her.
I hope that this is of assistance - please ask if you need further details
I am afraid only his mother can do that - assuming that she still has mental capacity