This situation is one of the very few straight-up bars to acceleration under the IAS.
I believe there is now an asterisk after that bar to having siblings in the same grade. There are some successful situations where a younger skipped into the same grade as an older sibling.
I don't doubt that it has happened and that it has worked for some families. But the version of the IAS that I read a year or so ago had it as an absolute bar. Which is not to say that it's not a good idea to go through the IAS for both girls. There are definitely some suggestions in there about what to do if you can't skip them because of this issue.
I think sometimes parents also get a weird idea of what "gifted" looks like when we have one of those really out-there kids. My younger DS had his parent-teacher conference yesterday, and he is doing really well, but "normal" really well, if that makes sense. (I was afraid that he was really delayed in writing, and he's actually a little ahead.) It's still kind of unsettling after older DD's wildly asynchronous profile.
