There are so many layers to a family that has children with special needs. Special needs affects everyone in the family not just the disabilities of an individual. This was definitely fascinating news to me when I heard it. I have shared it with a few other friends who have children in our similar situation, both typically developing and special needs, and they found this information to be true and helpful in understanding their own family dynamic.
Often times the typical sibling wants to be in charge of activities, tries to make themselves better than those around them or will argue. This is because so often the friends around them are talking about fun things they do in every day life with their own siblings, which is very typical. The typical sibling does not have these same experiences with their special needs siblings because they are unable to have age appropriate experiences. So, being saturated by hearing these circumstances, the typical sibling is hurt emotionally because they have a constant reminder of the experiences they are missing out on with their siblings because they have special needs. Often times the typical sibling also feels a self-imposed responsibility to defend the family honor by trying to prove the family is worthwhile too and that the typical sibling can make up for the things their special needs sibling is incapable of doing. This knowledge does not excuse bad behavior but lets the parents understand the emotions of the typical sibling and allows them to deal / teach them in appropriate ways.
Now, as a friend of special needs families how does that look? Do you avoid the family to not cause harm to the typical sibling? No, this would cause more harm to the typical sibling because they need and desire the opportunity to play with their typically functioning friends. The best thing friends of special needs families can do is treat them like they are "normal". Invite them to your house, don't be afraid to be seen in public with them (warning: there will be tons of looks and questions), understand their may be an extra measure of grace needed for the typical sibling and just be their friend.