My wife and I have a son with Autism and other disabilities. He is now 28 and holds down a full time job. We also have a daughter who is 30.
Our daughter has left home but our son still lives with us.
When they were young it was a real problem of how to deal with his Autism, and my daughter still feels a certain resentment that our son could almost "get away with anything" when they were both young.
Of course we used to say to our daughter "how would you like his disability" but when you are young you don't quite understand that.
My son could never deal with any change of plan when he was young. If we had arranged to go somewhere, but then had to change the place or time (or had to cancel the event altogether) he could not cope with that and we had to manage it very carefully so he did not "blow his top" due to plans being changed.
While my wife does of course love our daughter she does probably "care" more for our son, probably because she has had to spend a lot of her life taking him to hospital, to opticians and deal with all his paperwork (financial matters etc).
While our daughter lived at home, once she started work we made her pay a little for her keep (not much) but even though he is 28 my wife will not let our son pay for his keep (even though he has a full time job - though not that well paid to be honest) and this still causes some resentment from our daughter.
So how to deal with siblings when one child has special needs is difficult.
Maybe in your case the parents (or you) should take the daughter out for special treats WITHOUT the son now and again, maybe to the cinema, or theatre, or out for a special meals.
While out with her make it clear it is a "thank you" to her for understanding the situation with her brother.
If she feels she is being given these special treats it may make her feel better.