NonDYS siblings are not allowed to attend Summit, which can be a pain and does make things quite complicated. At this point DS5 is aware that his brother could attend the last 2 gatherings and he couldn't. I believe he fully expects to attend the next one. I hope he will because that means I will get to spend the entire time there as well instead of splitting the time between me and DH

We are going to our first summit, and this is what we'll be doing: switching off with our younger one. We also would expect her to be able to attend after that. She is always SO aware of everything. She complains bitterly every time her older sister gets to go to a party she can't attend. We are all working on dealing with this. She is too far away from grasping that famous "fair is not equal" deal.
I believe, as PP have mentioned that personalities and age differences matter here. I believe that a crucial variable is how far apart are the kids in question in terms of IQ/achievement.
Our younger one scored just below the mark in the WPPSI, given that English is her second language, we expect she would qualify after a year or 2 of full time school versus a couple of hours of preschool. However, it would be very hard for us to manage IF she doesn't qualify eventually. I think that if your YS' needs are much more extreme than siblings, then it is easier for everybody to accept and deal with it.