How should my child explain their sibling's disability to friends?

Whether and how much a sibling explains to friends is for them to decide. First convey "it's okay to talk or not to talk," and, for when they want to, prepare together a short, easy phrasing (for example, "my sister finds ○○ hard and needs this kind of help"). Convey too that "I'll ask my mom" is fine for questions they can't answer well, and that they can consult an adult if they get an unpleasant reaction.