So, here's the issue.
You owe capital gains tax based on the difference between the sales price and your adjusted cost basis. Your sales price is obvious, although you can reduce it by certain selling expenses, like transfer taxes, real estate commission, any inspections or surveys you might be required to pay in your area, and and sellers concessions (in other words, if the sales price was $250,000 but you paid $5000 of the buyer's closing costs, then your sales price is $245,000.
Your cost basis is more difficult to determine. For a gift, your cost basis is the same as the cost basis of the givers. That would be your parents. Their cost basis is what they originally paid, plus certain closing costs they paid at the time, plus the cost of any permanent improvements they made over the years. Certain carrying costs could have been capitalized (added to the basis) but you would need proof of this from your parents' tax returns.
If the property was bought by 2 spouses and one spouse died before the gift was made to you and your siblings, then the other spouse inherits a higher basis on the inherited share. But if both owners were alive when the gift was made, then you only get the gift basis.
So let's say your parents' basis on the date of the gift was $50,000. Your gifted share was $10,000 and your siblings shares were $40,000. If your siblings gifted you their shares, then your total basis is $50,000. If your siblings sold you their shares for $80,000, then your total basis now is $90,000.
Then you can increase your basis by any improvements you made after becoming the owner.
You may need to work quite hard to prove your parents' cost basis. If you are audited, the IRS will only give you whatever basis you can prove. The more basis you can prove the lower your capital gains tax.
