'Next of kin' has little legal meaning.
If you're considering who will inherit your estate when you die, you're free to leave it to anyone you like (irrespective of whether they're related to you or not). But you have to write a properly witnessed will. (Under certain circumstances a will can be challenged -e.g. if you try to disinherit someone who is dependent upon you - but normally your wishes stand).
Nominating one or more persons as 'next of kin' won't affect who gets your estate if you don't leave a will. Strict intestacy rules apply and must be adhered to.
If you're simply thinking about who should be informed if you're taken into hospital, or if you die, you can nominate as many people as you like. (They don't have to be related to you).
If you're thinking about who should arrange your funeral, you can request that one or more persons take on the task but nobody is ever obliged to arrange a funeral for anyone else. (Similarly anyone, even a complete stranger, is free to arrange anyone's funeral).
If you're considering who should be given control over your affairs if you become incapable of handling them yourself, you can nominate one or more attorneys to carry out those duties:
http://www.publicguar...rms/Making-an-LPA.htm
If I've not covered what you're really asking about, please post again with more information.
Chris