An IRA never gets a stepped up basis. The money was never taxed when it was deposited, and there was no tax paid on the gains, so everything is taxed when it withdrawn, even by the beneficiary.
I also want to nitpick here. If you are listed as your mother's beneficiary, then you now own a beneficiary IRA (inherited IRA). There are certain rules you must follow when making withdrawals, depending on your mother's age. However, if no beneficiary was listed, then the estate owns the IRA. Generally, the estate will cash out and close the IRA, and all the taxes are passed on to the heirs who divide the money. There is no stepped up basis.
Can you clarify if you are the beneficiary, or if the estate was beneficiary?
Other assets, like regular broker accounts, a house, or other investments, do get a stepped up basis. There is no tax on unrealized gains, it just goes away, by operation of law (that some politicians want to change, but that is a discussion for a different forum).
