DD2 is working on math a year above DD1. My oldest tells me she "brags" about her younger sister at school "because she is so cool to be doing that". I don't know how we got to this place, but they are pretty supportive of each other. That said, it's only subject acceleration, so they both get to have things they are good at. I don't know how it would be if DD2 was placed a full grade above DD1!