'Next of kin' has no legal definition. In some circumstances you're free to nominate anyone as 'next of kin', e.g.when going into hospital. (The person you nominate doesn't have to be related to you).
However if the question really means 'Who will inherit my estate upon my death?", then the rules if intestacy apply. See here:
http://www.tmjlegal.co.uk/wp-content/uploads/2011/11/Intestacy_Chart_web.png
(In the circumstances you describe, both parents inherit equally. The siblings get nothing). That, of course, assumes that you've not written a will, in which case you can choose who to leave your estate to. (It doesn't have to be to family members. None of my family are beneficiaries under my will).
[NB: I've assumed that you're in England, Wales or Northern Ireland. The intestacy rules are different in Scotland]