While I cannot address the US legal/tax issue, I have a smiliar distribution issue wanting to leave modest inheritances to my siblings with the bulk of the estate going to my children. The text of the will runs something like:
1. pay all taxes expenses and discharge all liabilities etc
2. allocation of some specific chattles
3. the first $x to be divided equally among my children
4. the next $y or, if less, the balance (if any) to be divided among my siblings and my children in equal shares
5. the balance (if any) to be divided among my children
Except for a few items under #2, nothing is left to my wife!!!
The other way to deal with this issue is life insurance policies which I have also done, but insurance is expensive and I will terminate the life policy once I retire.