I don't think anyone understand the older sister's frustration enough. All the sacrifices she's had to endure as an older sibling, all the nonstop hospital visits since newborn, all the stares from people in public, the talk behind their back, the dangers posing towards her sister, or the solitude of your sibling because of her handicap. Most don't have friends outside of their handicap social groups, when they yearn for "regular" friends too.
Not only that but there's even family rejection too. Whenever there's a family event, they're often left out by the family members, who'd rather socialize with the outside "guests," even after not having seen you over a year. Some rather not invite you at all.
To top that off, you have to live knowing she has to endure most of these things for the rest of her life, and there is nothing you can do most of the times, but to be strong for her.
The worst feeling as an older sibling is looking at your sad sister's face knowing she's being left out, rejected by the world, when she just wanted to be a regular person... it's crushing.