Consult with an estates attorney with litigation experience in order to evaluate the strength of any claim that you may have here. Bring the a complaint and any other documents that you wish to analyze.
Consult with an attorney and do not speak with your brothers until then.
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The previous answers are all great--you need to consult with an attorney. Having a POA enables you to act on your mother's behalf--but such actions must be for her benefit, not yours--unless that was part of her testamentary plan. On the joint accounts there may be a question of whether you really are the owner or the account belonged to your mother and your name was on it as a "convenience" to her, in which case the value might come back into the estate.
The others give good advice, especially Mr. Wurtzel. You need to see an attorney immediately. Based on your facts, you could possibly be civilly and criminally liable for financial elder abuse. You thought you were doing the right thing but that's irrelevant. If your mom wanted to equalize gifting, she should have done so while she had capacity or drawn up a will stating so.
You would have a right to claim fees for your care taking services, but you need to do that in the proper manner.
While the POA gave you rights to control, you had to do so as a fiduciary for your mom, not for your own benefit.
GET TO A GOOD PROBATE ATTORNEY IMMEDIATELY.
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