I was all gung-ho for helping a sibling group to stay together when I started this adoption process. I even looked down on those who would split up sibling groups, thinking it was better to keep such groups together.
I am much more skeptical of this now that I have gone through the ringer. In our case, having a family inside a family kept my children from healing. The oldest two might not have healed no matter what we did, but the youngest two got corrupted by the idea that our family was not a "real family" that subtly undermined our own family. The biggest problem is that we didn't realize that it was present until very recently, far to late to do anything about it.
While splitting up siblings and possibly reducing contact can seem cruel, it may ultimately be the best way to give these children a chance to heal.