I think you do have to carefully consider how it will affect the daughter not being skipped, since they will be in the same grade. Have you thought about talking to a counselor?

We made the decision not to grade skip one of our twins because it would have had long term psychological consequences for his brother. It is just obvious to us that it would cause problems, given their dynamic. To me, the need to not harm one child outweighs the advantage for the other.