Georgina,
Have you heard of Sibling Australia? Sibling Australia is a NFP advocacy group for siblings of those with a disability or chronic health condition. 
As a fellow adult sibling of a person living with a disability your story resonated strongly with me.  How many times does the NDIS or a Support Service check in on Tom, or your parents, but never asks about Toms sibling(s)?! ... Well, this network has been great for connecting with others who have similar experiences or are going through familiar challenges. 
Thanks for sharing!
Kind regards, 
JC