The term Next of Kin (NoK) is commonly used and there is a presumption that the person you identify as your NoK has certain rights and duties.
Health and social care colleagues should always consult the people closest to a person who lacks capacity to understand that person’s wishes and feelings to help them make a decision in that person’s best interests. However, the person identified as next of kin should not be asked to sign and/or consent to certain interventions (unless they have a legal basis for doing so). This is a mistake often made in many hospital and nursing or residential type accommodations, where family members are asked to sign care plans, end of life plans and other treatment options and provide consent which is not legally valid.