Most likely, the rest of the siblings are out of luck. The account would not have needed to be made joint, if there was a POA. That is one of the reasons to have a POA in the first place; so the ward's assets can be accessed, if needed, in order to pay expenses.
You have two possible avenues to challenge this. 1) If your BROTHER changed the account into joint ownership while acting under the POA, then this would have been inappropriate and a breach of his fiduciary duties. You could then ask the court to overturn that designation. 2) If your mother's Will specifically makes reference to the accounts and expressly states that they are being made joint, only for convenience purposes, then you may be able to have the account proceeds put back into the estate. I think the insurance is a totally lost cause, UNLESS your brother had the beneficiary designations changed to his name, acting under the POA.
You should consult with a probate litigation attorney to investigate this further to see if you have any basis for challenging this.
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I agree with Mr. Frederick as to the account. Unless you have strong proof that his name was added only for convenience, he is entitled to keep the assets in the joint account, since is on the account. note that you must be able to prove his name was only on the account for convenience by clear evidence, under Virginia Code Section 6.2-608, which is a fairly high standard. It is not absolutely necessary that the evidence be in the will, but you would need much more than just speculation. As to life insurance, that is not governed by the will, and instead the benefits are paid to the persons listed on a designation filed with the insurance company. The life insurance would only be paid to the estate if the designation of beneficiary form listed "my estate" or something similar, otherwise, life insurance is not paid to the estate.