When real estate property is willed to relatives, but not children or siblings, of decedent and they wish to sell said property, what are the federal income tax ramifications?

Assuming that none of the heirs have actually lived in the house as their primary residence -- the heirs will each pay capital gains tax on the differece between their share of the proceeds of the sale and their share of the cost basis. The cost basis is set equal to the fair market value of the property as of the date of the decedent's death. If there is a gain, you report it as a long term gain on schedule D. If it's a loss, you can not deduct the loss.

Just to add, the relationship of the heir(s) doesn't matter. The tax treatment is the same on inherited property whether the heir is a spouse or a non relative.