When we accelerated my DS in school we left him in the same level scout group as age not grade. He was able to maintain best friend contact once a week but I do believe if we hadn't done that #1 would have happened to him. He is in middle and best friend a fifth grader this year. They do chat and play video games over the Internet about once every two weeks too.
I was the older sibling in your scenario with a year younger brother who should have been grade skipped but never was. He was eventually accelerated a math class in high school by taking two classes at the same time and I was also taking that class. I told my parents that he had to have a completely different teacher if that was going to happen....and he did but he got the one teaching one section for gifted learners and I got the crappy guy who retired at Christmas/new guy after that was good. We had same teachers as we grew up...I just didn't want same teachers in the same year...like me in third period and him in fifth. It worked out. At first I wasn't happy but I realized after a while it wasn't all about me.
