The LLC members are her and my deceased father (2017).
That's a big problem. The Multi-member LLC/Partnership ceased to exist on the date of your dad's passing. That's because a partnership "MUST" have two or more members. You can not have a partnership or multi-member LLC with only one member.
The partnership ceased to exist on the date of dad's passing in 2017. So if it was not done in 2017, the 2017 IRS Form 1065-Partnership return needs to be amended and the partnership must be disolved as of the father's date of passing. All assets of the partnership are passed to your mother (the sole survivor of the partnership) on the K-1. The amending of the 1065 partnership return will result in a corrected K-1 being issued to your mother, showing the transfer of all parntership assets to her.
Then, your mother needs to also amend her 2017 personal 1040 tax return to include the corrected K-1, and to enter the rental property on SCH E on her personal 1040 tax return. When she enters the property on SCH E of her personal 1040 tax return, she will include in the cost of that property asset, the stepped up basis which is 50% of the FMV of the property on the date of your dad's passing.
Once that is done, your mom gets to amend her 2018 tax return. But it's not going to be as simple as importing the new data from the amended 2017 tax return. When you amend a return, importing from the prior year's tax return is just flat out not possible. So she will have to manually enter the rental property one keystroke at a time, along with the total of all depreciation taken in prior years including that increased amount of depreciation she took in 2017, on the stepped up basis. Any carry over losses (and there should be a *lot*) will have to be manually entered also.
Once the 2018 return is amended completely, then (and only then) can she even start her 2019 return. The 2019 return will then be able to import the correct data from the amended 2018 return. Just make *SURE* that you import from the .tax2018 file that is physically on her computer since that will be the amended one.
If you and/or her don't understand how all this stuff works, then you'll find the cost of professional help will seem like a pittance when compared to all the fines, back taxes and penalties that the IRS *will* assess if it's not done right. So professional help with this would probably be the better and less costly way to go for this year.
