I appreciate everyone's thoughtful answers. My brother started paying via monthly installments in Jan 2022 $1,443 at the end of each month.
He hasn't taken on any new partners as far as I know, and I want him to continue the business and do well. I wish the best for him, but not through me and my wife getting further financially/emotionally destroyed, you know what I mean?
About penalties and interest for the 2021 taxes, the accountant who bailed on us had said over the phone (of course not in writing) that she would explain everything to the IRS and that she'd send a screenshot of the filing confirmation she received, and then later said she sent the screenshot to my wife, which my wife never received. The accountant then said she was in the hospital and couldn't talk more, which at the time I believed her, and she's been ducking my calls and emails and texts ever since.
Regarding me not being able to take a deduction because he's my brother, I suppose it doesn't even matter in the end what my selling expenses were, if it all adjusts to zero anyway?
Also it was my understanding that we can put the entire sales price in the tax return of the year of sale, instead of doing it in installments. If it all zeroes out, then what difference would it make? I figure if I do it as one lump sum (even though I haven't received the full amount yet), then I won't need to amend my 2022 taxes which don't even mention the LLC.
