While siblings usually score within 10 points of each other, this is not always the case, especially when one of them is as extreme as your first child is. For example, in my sibling group, most are within 10 points of each other, but one is about 3 SDs above the rest. If only two siblings had been born (including the outlier), it would have looked even more divergent than your children's scores. It's only because there are more than two that it becomes apparent that one is an outlier. And let's not forget that your DD is still very bright, in the moderately gifted range.
Another possible angle is, were there any differences among the index or subtest scores that might make the global measure less complete?
And finally, scores in general are slightly lower on the WISC-V than on the -IV, mainly for norm obsolescence reasons. Your older child took the WISC-IV very late in its norm life, while your younger child took it fairly early in the -V's norm life, which would account for at least 4-5 (or even more, in this standard score range) points difference. 10 points drop in the GT population is pretty normal, moving from the -IV to the -V, according to the publisher's research.
