When you inherit stock you get what is a stepped up basis. that is the cost basis is the FMV of the stock as of the date of death.
so in your case, you need to get a copy of the estate tax return and gift tax return that was used to settle the estate. You can get that by contacting the IRS or the estates executor.
To figure the cost basis for a stock split, you have two options.
As of the date of the split:
Option one: Take the original cost basis (FMV as of the date of death) of all the shares, and divide it by the new amount of shares you hold to arrive at the new per share cost basis.
Option two: take your previous cost basis per share ) and divide it by the split factor (2:1). So in this case, you would divide the FMV per share as of the date of deathby 2 to get the new cost basis.
either option gets you to the same result. the only difference is what size numbers you work with.
If you use total FMV, it would look something like this.
1,000 shares whose FMV at death were 10,000 dollars.
With a 2:1 stock split, you would now have 2,000 shares worth 10, thousand dollars.
So 10,000 divided by 2,000 = 5 dollars per share
The second method starts with the per share FMV as of time of death,
In our example 10 dollars per share.
With a 2:1 stock split it would be 10 dollars per share divided by two = 5 dollars per share.