How old are your sons? Could you request an eval for the younger one just to put your mind at ease? Does the younger one spend time around neuro-typical peers? How does his behavior compare to his typical peers?
Could these behaviors be rewarded in some way in your family that might cause the younger child to continue them? Are they working for him in some way?
My older child has sn and I worry about my younger child from time to time too. Last year, we had soooo many issues with the older child that I completely lost track of what was going on with the younger one. I sat in a parent teacher conference and just asked questions -- worried that any thing could be going on with her and I wouldn't have a clue because her sister was taking up all my time and energy. She was fine.
Still, if you are concerned, finding the right person and asking questions is most likely to the quickest way to get it resolved. It's helpful when someone who knows your child tells you they are fine!
BTW, I think it is really good for typical sibs to have time away from their families, and I'm a fan of preschool or school. It lets them get out of that role for awhile.