(2-part post):
'Next of kin' has very little meaning in law. You can actually nominate anyone you like (including a total stranger who you've never met) to be your 'next of kin' for most purposes.
If your parents have written wills, the executors of their estates will be named in those wills. There is no reason why the executors should be related to your parents and (unless you or your siblings are legally dependants of your parents) there's no reason why you should be among the beneficiaries of the wills. (My will names five beneficiaries, two of whom are also named as executors. None of them are related to me).
If your parents don't leave wills, someone will have to apply for 'letters of administration' to distribute their estates. There is a 'pecking order' defined by law. When the first partner dies, their spouse heads the list of people entitled to apply for letters of administration. (If the estate is worth less than a certain amount, which I believe is currently �125,000, it passes automatically to the spouse anyway). When the surviving partner dies, their children head the list of people entitled to apply for letters of administration but the oldest child is treated in just the same way as the youngest. (Any one of the seven children can apply for letters of administration, or a group of you can do it together).