The total basis for each remaining sister when they sell the house would be 1/3 of the FMV at death plus 1/2 of what they paid to the third sister for her share.
@AlanT222 I think your train of thought may have been a bit skewed here. Happens to me at times too. The cost basis would be 1/3 of the FMV at death plus THE TOTAL AMOUNT EACH PAID to the third sister for her share. Not one half of what each paid to the 3rd sister.
