In my family, most of us siblings and cousins are around the same level of giftedness as at least one other cousin or sibling (all GT to Profound GT). Some research on siblings shows that two siblings usually are similar in intelligence, as intelligence is a highly genetic trait. If one child is comfortably above the cut-off, the other is likely GT, as well. As for the school, it's probably too early to tell if it would be as good of a fit for the younger one as it is for the older one.