You may be able to treat this as the sale of investment property, rather than sale of a home. As investment property, you can take a tax deduction for a loss (sale below cost basis) when you can't take a deduction for a loss on personal property. The key question is did any of the siblings live in the home or use it for personal use.
For example, if sibling A lived in the home and took care of mom, but siblings B and C did not, then A can't deduct the loss on their share but B and C can deduct a loss. Or, suppose mom had a house on a lake, and all the siblings took one last vacation at the lake before selling. That would make the sale personal.
Also, if you received a 1099-S at the closing, you must report it on your tax return even if there is no taxable income or deductible loss, because the IRS will be looking for that 1099-S to be accounted for.
