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I am sorry to hear about your loss. I am sorry to say that yes, she can do this.
This is because money from a pension plan (401k, IRA, etc) is outside of probate and is not subject to probate. The designated beneficiary of the pension CONTROLS and if the Will is different, then the Will is overridden when it comes to the pension.
In other word, whatever was written in the Will does not matter when it comes to the pension - the proper beneficiary is that which is listed in the pension plan record, only.
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