This situation is one of the very few straight-up bars to acceleration under the IAS. She could be a great candidate in every other way, and it would still say not to skip the younger daughter in a way that puts her in the same grade as the older one.

That said, doing the IAS for your older daughter and thinking about whether the grade skip is good for her is a reasonable thing to do. But try not to let the prospects for your younger daughter be a thumb on the scale in favor of moving them both.