How lucky to have children who are so gifted - congratulations!
We had the same problem in our house growing-up. I was the one drawn into the gifted magnet program in 3rd grade, my sisters were not. At one point, I was 2 grade levels ahead in math - the same grade as my sister who couldn't handle it. So, she took summer school to get ahead of me and not be faced with being in the same class as her younger sister.
That's something I would think about - not only the impact on the younger daughter, but also how it might affect the older daughter.
If your younger daughter is OK with it, I wouldn't discourage it. But, the teenage years are tough. She'll be in a much different place from most of the kids in her class, and that appears to be where the advancement can be most difficult for children who are advanced into the next class.
One thing I did in high school to challenge me was to start taking classes at a local community college. We had an accredited University closer, but as a former professor, my father thought the teachers at a community college would be more interested in my development than in their research.
I'd also check to see if any near-by colleges/universities have programs for gifted children. I know Ball State University (Muncie, IN) has a program where teenagers are basically enrolled in college as young as their sophomore years. It's brutal, but for the truly gifted students, it gives them the opportunity to be in an appropriate environment.
Finally, I'd also recommend perhaps taking it upon yourselves to find a tutor/activity that really challenges her outside of school. If it's science, math, dance, etc. perhaps the extra attention outside of school will broaden her horizons, keep her engaged in something outside the boundaries of school and prevent the social backlash of moving her up inappropriately.
Good luck!