It kinds of depends on when the various paperwork happened. If the estate owned the house, paid for repairs, and then sold it, the estate would file a tax return and issue a K-1 to each beneficiary.
If the deed was transferred to the 3 siblings, who then paid for repairs and sold the home, then each sibling will report the transaction on their individual return using 1/3 the cost basis and 1/3 the proceeds.
In either case, the starting cost basis is the fair market value on the date the prior owner died. Improvements add to the cost basis, but repairs do not. The difference is that a repair restores the property to as-was condition while an improvement adds value or extends the useful life of the property. Replacing the roof is an improvements, fixing one leaking spot is a repair. Cleaning and painting is a repair, not an improvement.
The selling price can be reduced by certain selling costs, like advertising, certain taxes and fees, and the real estate commission. "Staging" counts as advertising as long as no changes are made to the home after the staging is removed. Adjustments to basis and selling price are listed in publication 523.
https://www.irs.gov/forms-pubs/about-publication-523
Depending on your selling expenses, you might even have a deductible loss, as long as you treated the home as investment property and did not have personal use. For example, if this was grandma's lake cottage and you all used it for one last summer before selling, that was personal use and you can't deduct a loss even if you show a loss after figuring your improvement expenses and selling expenses.
