The expert you talked to above is no longer online, while I see you are, so if I may, I would like to answer your questions and clarify the issues.
For 2013, the capital gains tax rate is 0% if you are in the 10-15% tax bracket. It raises to 20% if you are in the 25, 28, or 33% tax brackets. If you are in the 35% bracket and your AGI is over $200k for single, $250k for married, the rate is 15% plus a 3.8% surtax. If you are in the 39.6% bracket, the rate is 20%.
The 25% rate that the expert above mentioned is very limited, and only for certain rental properties that were depreciated. If your mom rented the land to a farmer, land is not depreciable, so the rate should not apply if that is the only rental income.
You cannot deduct the cost of the improvements. What you have to do is to take your mom's original cost, as the expert above stated, and add the cost of any improvements to it. You are correct when you ask about proof for "guesstimates" that the expert above suggested. The IRS says if you can't prove basis, the basis is zero. However, there are many ways to prove basis! It may take some digging, but your mom would have had to get building permits to construct the home and for the improvements. The permits require the contractor or owner to list the cost of the construction, so that the assessor can more properly assess tax.
One question, however, that may make things easier - what about your father? If he owned the property and then left it to your mom when he passed away, you get a "step up" in basis to the FMV at the time he passed away. This may be easier to determine, as you can get comparable sales from the time and use those for establishing the cost basis.
The rules say that when someone gifts you property, as your mom did when she went into the nursing home, it is not an "arms-length" transaction, so you can't use that for establishing basis. You therefore get your mom's cost basis. Which, as I mentioned above, could be more than cost if your father owned it prior to your mom.
You asked about your sisters. You can allocate part of the sales price to your sisters, and they can report it on their tax return, although if the property was deeded to you, and you gave part to them, it could be construed as a gift, and there could be gift tax implications for you. Since capital gains is a flat tax on the net gain, you can easily allocate part of the tax cost to your sisters. If you calculate that the gain is 15% of the net, you can simply have your sisters pay their share. However, this would mean that you have to give them some of the sales price first.
As far as you wanting to deduct the cost of improvements, such as the garage, outbuildings, etc., these can't be deducted on your tax return. They get added to the basis of the property (which I discussed above) and reduce your capital gain.
I hope that I have added to your information and answered your additional questions. If you have any more questions or need clarification, please feel free to ask away!
Have a great weekend, or what is left of it!