Will we have to pay taxes on the land left by our mother?

I'm assuming all the final taxes (income and estate, if any) were paid on your mother's estate. I'm also assuming that the property taxes have been paid and are up-to-date. If this is true, then you can claim a stepped up basis in the land to the time when your mother passed. Any gains from that time would be considered income and subject to income tax.

I assume your mother's taxable estate was not large enough to require any payment of an estate or inheritance tax. The value of her taxable estate would have to be in excess of $5 million to result in a federal estate tax. The Illinois state inheritance tax comes into play at a lower figure with a more complicated calculation but for simplicity lets assume the value of her taxable estate was less than $3.5 million, thus no state inheritance tax. You will have to pay a capital gain tax based upon how much the sale price exceeds the costs of sale plus the value of the property at the time of your mother's death.

Any gain in value from Mom's date of death to the date of sale is taxable as a capital gain. Talk to an accountant.