jennyjoan, it is really only an issue when authorities need to know who to contact such as the hospital when a person is admitted, the police when somebody is arrested (maybe) so there is a point of contact, someone who the patient is happy for private matters to be discussed with.
My GP has my nominated next of kins details, one is my wife and the other is a trusted friend because if I am in a car accident of plane crash it is highly likely my wife will be with me.
In a will, the next of kin doesn't come in to it. You name the beneficiaries who don't need to be family and the executors, who can be anyone you want.