I haven't read all the advice in detail here but in skimming it, there's some excellent advice.
I can't really add much else except to say that DD2 was in and out of hospital from birth and a lot of attention was on her because of serious health problems which went on throughout DD1's and DD2's childhoods and beyond. Funnily enough, it was DD2 who was jealous rather than the other way round - she resented DD1 for being popular, having friends and no health problems or a disability. We hadn't expected it to be that way round (how stupid, I know!) - DD1 seemed to accept that DD2 needed our attention and didn't display OBVIOUS outward jealousy. However, she was a very lively and demanding little girl and very assertive! We tried to ensure that DD1 had her special time alone with DH or me - in fact, we did that with both of them. This gave them a break from each other and time with one parent. It wasn't always ideal, but it was the best we could do. In hindsight, we expected far too much of DD1 - she always seemed mature beyond her years, but having discussed with both of them how they saw their childhoods, we realise we didn't acknowledge enough that DD1 might have difficult feelings she needed to share or give vent to. It was always like walking a tightrope, getting the balance right - it probably always is, but when you have any child with special needs, that can present so many more difficulties.
It was DD2 who developed serious mental health problems, probably a part of her syndrome but we will never know for sure. Thankfully we all lived to tell the tale (only just, in her case) and DD2, though living with a serious disability, is now coping well, did well with therapy and has a beautiful baby girl of her own - to whom she is a wonderful, loving mother.
I think what I'm trying to say, in a roundabout way, is that it helps to acknowledge and air difficult feelings in siblings, but you can't protect them totally from pain and it's by learning to deal better with painful feelings, as well as the good, that emotional maturity and emotional good health is nurtured.