I've read the same - that siblings are typically within 10 or so points of each other.

Honestly, I don't think that's the case with my two... my son is at least highly gifted (hard to say with his 2e-ness) and picks things up the very first time he learns about them... while my daughter... she's a good student, meets the school's goals, but doesn't really seem to go much beyond that, and often has a difficult time learning new things. I'd say she's maybe mildly gifted.

~amy