Is there an option where she can use EPGY or ALEKS or Khan Academy or some other online curriculum on a classroom or library computer for subject acceleration while staying with her classmates for things like PE, art, etc.? This might be the best of both worlds, and would avoid the commute.
Is staying in the same school with her brother important to her, to him, or to you? It can be difficult to be the younger sibling of a very bright older sibling - sadly, teachers often have expectations of younger sibs that are based on their impressions of what the older ones were like, and your son might not appreciate growing up in his sister's shadow at the school.
It really is important that kids learn to work hard - not just complete busy work, but actually exert themselves to master the material. Otherwise, when they finally hit material that *is* at a level of challenge, sometime in college, say, they won't know how to study and organize their thinking, and they will have a very difficult time of it.
I can't begin to tell you what the right solution is for your family, but I hope this gave you some ideas that may be helpful in clarifying your thoughts.
