From my personal observations of regular siblings raised apart, there are a few problems that come up when they are reunited and studied.

My mother was a middle child of five who was put up for adoption (her birth family was facing an economic hardship and they couldn’t afford to have another child around the time she was born).  The rest of her birth family remained together, and after the death of my mother’s adoptive parents, she set out to find her real family.

They were reunited five years ago by mail, and shortly afterwards, my mother and her long lost brother and sisters participated in a study of separated siblings.  Unfortunately, she had found that the rest of the family lived a pretty hardscrabble life, whereas my mother was more spoiled and had more opportunities.  This caused a tinge of resentment among her siblings.  There is also the idea that when everyone is joined together, they THINK they know each other when they really don’t and I’ve seen this to cause hurt feelings.

There is also the matter of finding out about family genetic health issues, and learning about what you may be predisposed to can be disconcerting.  Lastly, there is the freak chance that when four sisters get together, they could all show up wearing the same hideous pink shirt and blue jean combination (If only I could find the picture).

IMAGE CREDIT (and topical coverage):
Twins separated at birth reveal staggering influence of genetics