Sibling rivalry is largely based on the perception of time and attention. No matter their ages, siblings seem always to have one eye on whether things seem fair. Often, there can be resentment toward a child with special needs; it may feel like Mom is paying more attention 95 to the sibling who needs a little more help.
It is important to spread the time and attention around. It is also important for your child to devote some time and attention for his siblings. While you don't want to overload activities, it is a good idea to have your child attend at least some of his siblings' activities (games, programs, etc.).
Make sure that all of your children are on the family team, so to speak. Everyone should encourage the others. Everyone should be excited when someone accomplishes a goal, and everyone should be sympathetic when things don't go right.