When there are neurological quirks in play, that can change the game. Some children's hospitals run "sibshops" that help children ages 6-12 learn about their sibling's special needs and gather coping skills. (One should check content for appropriateness, of course.)

When the sibling is undiagnosed, it's harder to offer a handy label, but it can be important for everyone to realize "this is how your [sibling] is" and "this is what [sibling] needs" and "I will always try to make sure you both get what you need."