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If the monies you received were paid to you as a result of instructions from your father's will or if your mother simply gave your brother and you the money it is not taxable to either of you. Cash money inherited is not taxable nor are gifts to the receiver.
Your mother will be responsible for the taxes that would be due and the reporting of the distributions to her .
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Her tax rate will depend on her filing status, total income, and deductions. It is safe to assume though that your mother is looking at over 35% plus.
Here is a tax rate schedule for 2010 taxes:
Unless your mother remarried she will most probably need the first box for single and the last line.