You will not be comparing apples to apples, though they should still be in the same ballpark. There are actually a few issues in comparing:
1. Different overall test structure: The WISC-V has five primary indices, consisting of Verbal Comprehension, Visual Spatial, Fluid Reasoning (these two used to be combined in Perceptual Reasoning), Working Memory, and Processing Speed. There are also a bunch of ancillary indices, which may or may not be administered, including measures of quantitative reasoning, pre-literacy measures, etc.
2. Different subtests contributing to the index scores: Each index is calculated from two subtests, instead of two or three, as on the WISC-IV. This means intra-cluster subtest variation has the potential for a greater impact. There are also new subtests in the primary indices. For example, Visual Spatial consists of the old block design (new items, of course) and the visual puzzles subtest, and Fluid Reasoning is composed of the old matrix reasoning and the new figure weights. Working Memory now has one visual span and one auditory span measure. The subtests themselves are not truly new, as the majority of them have been in the WAIS or WPPSI in the past. Just not the WISC.
3. Equally important, the norms are much newer than WISC-IVs administered three years ago: Her brothers took the test on nearly-decade old norms (WISC-IV was published 2003, so the norms were probably collected in 2002, but they took the test in 2012, so ten years old). She is taking the test in the first year of publication, on norms that are less than a year old. According to most estimates of the Flynn effect, the average score inflation due to norm obsolescence is about 3 FSIQ points per decade--but the rate of inflation is believed to be significantly higher for the high cognitive population.
4. And on a side note, you probably already know that most siblings score within 10 points of each other.
