I think the first thing to consider in answering this question is the varying ages of the children when their parents died. Our narrator, Kate, who was just seven at the time, would have suffered an even more profound loss if she had been separated from her siblings—even if this would have led to a more stable life. Baby Bo, who would have grown up with no memory of life with his siblings, may have faced behavioral challenges. The young children might have wound up in the foster care system, which could have impeded their personal growth.

It would have been different, on the other hand, for the older siblings. Luke wouldn't have had to give up his college scholarship to get work in order to provide for the family. If the children had been separated, Matt also would have gotten to pursue his education. This in turn would have meant that Kate did not have to suffer the immense feelings of guilt that she had later in life as a result of her having been able to pursue their shared passion for science while her brother had not.

In a nutshell, I would imagine that if the children had been separated, they would all have led drastically different lives. Without knowing what those lives would have been like, it is difficult to imagine how that alternate reality would have defined their growth. I would always argue, however, that the best thing for siblings is to be kept together in the aftermath of such a tragedy.