This is too complicated to provide reasoned analysis. Sit down with a probate attorney to discuss all of the possibilities.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
I assume that you are asking because you are the brother that was not on the account. Unfortunately, I think that you are out of luck, unless the Will specifically mentions the account and states that it was set up the way it was "for convenience purposes only." Your argument is even tougher if it was POD or TOD. If your brother is intent on keeping this account, I do not see much chance in your overturning it, UNLESS it was set up that way BY your brother, while acting under a POA form.
The Will applies only to probate assets. The account, as set up in your hypothetical, is not a probate asset, and would not be controlled by the Will.
So in your example, the appropriate legal result is 3/4 to one brother and 1/4 to the other. If this was not the parent's intent, then this is another situation where proper legal advice could have avoided a completely unintended result.
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Based upon the limited information in the question, the two brothers would each be entitled to $50,000 from the estate.
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