You have to apply to the Court of
Protection to become what is known as a Deputy.
In your sister was capable of granting
consent, then you could have applied for a lasting power of attorney, but that
is not an option in this case.
Deputy ship will then give you
authority to manage your sister's affairs in total.
There are two types, and you may want
to apply for both. Finances and also health and welfare.
Unless you know what you are doing, you
might want to get a solicitor to deal with this for you. Although there is no
reason why you cannot do it yourself, the paperwork is full of pitfalls for the
unwary. It takes several months (there is no shortcut) and a solicitor will
charge about £2000.
There is some reading here https://www.gov.uk/become-deputy/overview
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