If the buyout of the siblings was part a sale.....$3,000 each.....and part a gift then this applies:
§ 1.1015-4
Transfers in part a gift and in part a sale.
(a) General rule. Where a transfer of property is in part a sale and in part a gift, the unadjusted basis of the property in the hands of the transferee is the sum of—
(1) Whichever of the following is the greater:
(i) The amount paid by the transferee for the property, or
(ii) The transferor's adjusted basis for the property at the time of the transfer, and
(2) The amount of increase, if any, in basis authorized by section 1015(d) for gift tax paid (see § 1.1015–5).
So if
@USM Professor paid $3,000 each and $5,833 was the gift portion then the gift portion would be used as the basis because it's greater than the amount paid.
