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You asked for IRC references - that I provided with extracts - yes - the legal language is slightly different that plan English.
The stepped-up basis is the fair market vale of the property at the time the decedent died.
You calculate the capital gain as (selling price) - (basis)
If you are not comfortable reading IRC - I suggest to use URS publication which are written in relatively simple terms
Specifically for your situation - see IRS publication 551 - www.irs.gov/pub/irs-pdf/p551.pdf - see page 9:
If you inherited property from a decedent who died before 2010, your basis in property you inherit from a decedent is generally one of the following.
The FMV of the property at the date of the individual's death.
The FMV on the alternate valuation date if the personal representative for the estate chooses to use alternate valuation.
The value under the special-use valuation method for real property used in farming or a closely held business if chosen for estate tax purposes.
The decedent's adjusted basis in land to the extent of the value excluded from the decedent's taxable estate as a qualified conservation easement.
In your situation item #1 is used - and that is so-called "stepped-up basis"
Let me know if you need any help.
Under IRC section 1014(b)(9) - the part of the property included into your father's estate would have stepped-up basis.
Because Massachusetts is not a community property state - 50% of the property value would be included into your father's estate and you need to use the stepped-up basis for the 50% of the property.
So your accountant was correct.