Complex sentence: She was actually one of a pair of ischiopagus conjoined twins one of which was headless because its head had atrophied and chest had not fully developed in the womb, causing the appearance of one child with four arms and four legs.
Split into simpler sentences: She was actually one of a pair of ischiopagus conjoined twins. One of which was headless because its head had atrophied and chest had not fully developed in the womb, causing the appearance of one child with four arms and four legs.