Your cost basis is not what they paid, it is the fair market value on the date the previous owner died. If you sold within a short time of their death, and the real estate market was not super crazy, it is likely that the value did not change much, so you sold the property for the fair market value. That means you have no capital gains to tax.
If the market did move, then you will owe capital gains tax on the difference between the selling price and the air market value on the date the previous owner died. You may need to get an appraisal to support your valuation. (In your case, each sibling reports half the basis, half the selling price, and half the gain.)
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