It seems that Darla and Mark Pritchard were having trouble conceiving...
Two attempts at in-vitro fertilization had failed, and they were about to make a third try when Darla's twin sister, Dana Johnson, offered to be a surrogate, to up the odds of success. Darla and Mark took her up on the offer.
It turned out both women, 39, got pregnant when Mark's fertilized eggs were implanted.
On Feb. 27, Darla gave birth to twins, Mallory and Wesley and, six weeks later, Dana had a girl, Mattie.
They're considered triplets and biological siblings. [Link]