From what I’ve read in real estate columns, the issue from a sales perspective is that you’ve basically gotten her basis in the property.
Say she spent 100,000 on the condo, and put another 20,000 in improvements. If the condo sold for anything over 120,000. the difference would be a capital gain (and therefore taxable).
I don’t know why a retroactive appraisal would be needed, but you can probably find out her purchase cost - either from her original paperwork, or the taxing authority where the condo was located (our county has all sales history). And of course you’d need to dig through her records to find out improvements as Dracoi said.
There are exclusions to what is taxable, provided you lived in the property. For example we sold a townhouse 10+ years ago, and made a lot of money on it - but because it was our principal residence, 500,000 was excluded from capital gains (our profit was nowhere NEAR that, so we paid no tax). I don’t think any of you lived in it, so this wouldn’t apply.
As it seems like you’ve heard, deeding the property to you and your sisters wasn’t the best deal from a tax perspective.
The taxes you paid out of settlement proceeds: I would bet they are not income taxes, but rather a pro-rata share of that year’s property taxes, or other recording fees (but I admit I could be wrong). Check the paperwork. As such, that wouldn’t have anything to do with your income tax. It might be used to reduce your net profit - certainly any real estate commissions would.
From an income tax perspective: you might well need to file a return in the state the estate existed in (you mention Maryland: was that where your mother lived as well?). When we sold my mother’s estate, we had to pay capital gains on some stock she owned and some interest income, to Pennsylvania (where she lived; we lived in another state). We also had to include that income on our Federal return and our home-state return (though the state tax was offset by what we’d paid Pennsylania).
I concur that this one would be worth taking to a real accountant, vs. trying to do it yourself.