A living will is a legal document in which you state the kind of health care you want or don't want under certain circumstances. It doesn't have beneficiaries.
You may meant - a health care proxy (or durable health care power of attorney) - a legal document in which your mother named you to make decisions about her health care if she become incapacitated.
It is possible to have both - a health care proxy naming a person to make the decisions, and a living will to help guide that person in making the decisions.
Your mother is not required to inform you about such decision in advance. You are not required to accept the responsibility either and may reject them any time.
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