when helping support siblings, i have included them in referrals to social work and psychology so that their feelings may be heard and attended too.
If they come withe the family for follow up, i make sure to make a fuss of them and ask how things ar going and offer them an activity to do while im consulting (drawing on the chalkboard, helping me wake weight and height and pack any supplements needed).
the small act of giving a sticker and asking how their day is lgihts up their faces!
when i ask of therer are any questions at the end of the consult, i make sure to extend the question the whole family. often, they have so many questions, not necessarliy related to the sick child, but i can see the relief when they realise that someone is willing to listen.