My dc's have two cousins with SN whom they love very much and play with a lot, seeing them most weekends and in hols.
When we are all together I find that the grown-ups sometimes get frustrated/irritable with the cousins, sometimes IMO unfairly so, and I feel that this is rooted in an inability to accept the situation. The nephews do have some challenging behaviours but are not difficult children IYSWIM - just developmentally behind and probably their parents and grandparents find it difficult being faced with how behind they are and are possibly a little bit embarrassed.
They are very loved and cared for but I just wondered if this was a common reaction in this situation.