Hi raja
All I can say is to not get caught up in peoples personal experiences, like you said every child and every situation is different. The trouble is you don't know how the road will go until you are travelling along the way.
The majority of 'stories' out there will be positive, however you don't know how positive your own story will be until you are writing the book.
There will be an impact on the siblings regardless of whether the child be DS or not severe or NT, all you can do is to be there for them all.
It also doesn't matter whether the child is mild, moderate or severe you will love the child regardless as you do your others, you will find attributes and qualities you never knew you had. There will be all different times and all different emotions along the way but you will find the strength, the support thats needed (now all those who know me are thinking well you haven't, but realistically I have as I'm still here!!).
Yes I am a parent of an 11 year old child who has DS, is at the severe end and is complex and challenging. Apart from his heart condition which was diagnosed at birth before the down syndrome his needs/issues/problems/diagnosis has accumulated as the years have progressed. His siblings, although they love him do feel they lacked attention, support etc due to him taking a lot of our time. There is times I don't cope and my son has never had the correct support, however I am in a minority.
Please feel free to ask anything. Please focus on the beginning of my post, arm yourself with the facts of what can coincide with any LD, not just DS. Only you know what is best for you and your family.
Take care x