I realize people say things out of nervousness. Lord knows I've been known to say the dumbest things. But when it comes to the death of any child, regardless of the circumstances or disability, the sorrow and pain is the same for any parent and sibling. Sometimes hugging the parent and saying "I will miss her, too" is just enough. Take over a meal or cookies for the siblings, but please, please, please, don't try to minimize or maximize the significance of someone's life due to their disability. People are not their disability labels. Regardless of how difficult disability might look to someone from the non-disability community, that person will be missed all the same with or without their disability.
Just be there to comfort and listen.