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You are correct in much of what you say.
If you take the money now it is fully taxable although you would not be subject to any earlier distribution penalties even though your brother is under 59 1/2.
You can do inherited IRAs and spread that taxation out. However, since your mother was over 70 1/2 (I assume considering your age) and taking out RMDs you are required to take distributions out over her life expectancy, not yours. You only have the option over yours if she were not taking RMDs.
This decision must be made before the end of year and the IRA must be set up in that manner. If not, you lose the ability to do life expectancy and must instead liquidate it within 5 years.
You do also have the option, if it interests you at all to do a Roth conversion, pay the tax on it all now and then no RMDs are required.
Hope this helps. If you have any further questions, hit the need info button and ask. If you are satisfied with the answer please hit the green accept button to close out the question and allow me to receive payment.
If the owner died on or after his or her required beginning date, and you are the designated beneficiary, you generally must base required minimum distributions for years after the year of the owner's death on the longer of:
Your single life expectancy as shown on Table I, or
The owner's life expectancy as determined under Death on or after required beginning date, under Beneficiary not an individual , later.