Can we start by getting a few more details please?
You refer to 'executors' but you've made no reference to your mother leaving a will. Further, while a will can name 'reserve' executors (in case an executor is unable to act, or simply doesn't want to do so) there are no such people as 'first', 'second' or 'third' executors'; they're all equal.
If your mother left a will we need to know its provisions, who was appointed as executor(s) and whether probate has yet been sought or obtained (through an application to the Probate Registry).
If your mother didn't leave a will then the laws relating to intestacy apply and here estate MUST be distributed in accordance with this flow chart (where 'you' refers to your mother):
http://www.tollers.co.uk/system/docs/178/original/Rules%20of%20Intestacy%20Flowchart.pdf
Under such circumstances there is no 'probate' and no 'executors'. If your mother's estate (including, for example, the insurance) was valued at over £5000 then the relevant people (which appears to be you and your siblings) MUST apply for 'letters of administration'. Once the grant has been received, you'll all be 'administrators', not 'executors' and (again) there will be no '1st', '2nd', '3rd'; you'll all have joint powers.
Where letters of administration are required (because the estate is valued at over £5000), banks and insurers can't hand over the funds until such time as the grant of administration has been obtained.
Where letters of administration are not required (because of the low value of the estate), banks and insurers MAY release the funds to those who are administering the estate (which would still be you and your siblings, with JOINT rights - NOT '1st', '2nd', '3rd'!) but they can also still insist that letters of administration are obtained first.
I was going to write more but I've realised that there's little point in going further until we know whether there was a will and, if so, what it actually said. If there's no will, is the estate valued at more or less than £5000?
Chris