Most likely, the IRA custodian will have language in their agreement with your mother that if she fails to designate a beneficiary, that the default beneficiary of her IRA is her estate.
If her IRA is an estate asset, then it would pass according to the terms of your mother's will. Since she probably did not specify a disposition for her IRA in her will, it probably would pass according to the residuary clause, which probably has the two of you splitting things 50/50.
If the above is all true so far, it's likely that your sister closing the IRA account was not proper even though she was the executor, because she didn't properly dispose of assets. If so, you, as the beneficiary, probably could sue your sister for her failure to act properly. This could, of course, lead to family discord.
What should have happened, anyway, is that your sister, as executor, should have worked with your Mom's IRA custodian to set up two inherited IRAs (one for each of the two of you as beneficiaries) and then funded it 50/50 from Mom's original IRA. The two of you would have to work together to take your Mom's 2021 RMD before 12/21/21. If your Mom was already subject to RMDs, I think you would then be able to take RMDs based on your Mom's life expectancy. This would have stretched out the RMDs (and thus the tax bill) over another decade or two. If your Mom was not yet subject to RMDs, then you would each have 5 years to empty the IRA. See
https://www.wolterskluwer.com/en/expert-insights/secure-act-changes-distribution-requirements-for-some-individuals under "Not a designated beneficiary"
The mildly good news is that closing out the IRA as your sister did should count towards your Mom's RMD for this year, and since she took 100% of the IRA, then your Mom's RMD for this year should be satisfied and there will be no excessive deferral penalty.
Assuming your sister closed out the IRA after your mother died (right?), I think that entire distribution is probably taxable to your Mom's estate on your Mom's 2021 estate income tax return (Form 1041). And it'll be less tax overall, but probably still a boatload of taxes, for the estate to pass through that income from the IRA distribution to the two of you via a K-1. Bottom line, you'll have ordinary income equal to half of that IRA distribution added to your 2021 income taxes, which will be taxed on top of whatever other income you have this year.
The distribution should not be reported on your Mom's final income tax return if it happened after she died. Basically, anything up to the day your Mom passed away is reported on her final income tax return. Anything that happened after that in her name (i.e. before distribution) would be reported on her estate income tax return. Anything that happens after distribution would be reported on your and your sister's income tax returns.
Note that an estate income tax return is done on Form 1041 and is not the same as an estate tax return that would be filed if your Mom passed away with more than $11.7M in her taxable estate. A Form 1041 estate income tax return is due if her estate earns more than a few hundred dollars (I think it's $600, but check the Form 101 instructions).
(The estate might be able to keep the IRA distribution as income on the estate income tax return, but estate income tax rates are much higher, and brackets are much smaller, than for individuals.)
Really, the attorney should be advising your sister on what she should have done with the IRA, and the CPA should be advising her and you on the tax consequences and tax reporting. I'm not sure why that's not happening, but that's sort of another problem that the two of you may want to figure out. I could imagine them not being happy with what your sister did, but they are professionals and should not abandon her as a client just because she made (apparently) a mistake. Unless she's not paying their bills or she is doing something else majorly wrong.
HTH. IANAL. YMMV.