My kids are a year apart in age. My daughter is older as well and very different (they don't even appear to be siblings) but none of the teachers have ever really commented on it or mentioned my DD to my younger child. At the most, they asked him how she was doing. Of course, they were only in the same school for about a year and a half. They shared the same kindergarten teacher but not first grade. I wouldn't worry about it until/unless you run into problems. You can comment to the teacher that he is very different from your DD but has a lot of strengths that she doesn't have.
