Probate is a public process. First order of business is to check with the probate court and determine if probate was opened in the county in which the decedent lived at the time of death.
If so, get a copy of the probate file. If the will was filed, a copy will be there. If probate was opened under intestacy, then a will was not filed. Unless you know more about this will and how to locate a copy of it, intestacy may be the way the estate proceeds.If you know what attorney drew up the prenuptial agreement, you might contact him or her and see if s/he ever did a will or recommended an estate planning attorney to his client. Follow up on any leads that you develop.
If probate was not opened, then open it under intestacy and seek administration of the estate. She will object, of course, but even if she winds up as the personal representative of the estate, you can seek to hold her accountable before the probate court.
This one may not be a good DIY legal project. The estate can use estate funds to defend against claims against it. You likely will have to pay a probate attorney with litigation experience to represent your interests. These cases are not generally done on a contingency basis. It can get expensive so you and your siblings may need to weigh the potential costs against what you hope to gain.
In most states, surviving children do inherit under intestacy and the prenuptial agreement may matter, even if he never got around to doing a will.