@Viviennemary
Now youve explained its quite clear the mil is concerned for the children. Not only do they have one new baby to deal with but two. Why do folk do this to children. They are now pushed out twice. Very sad.
Really??? 
People wonder why SM's get so exasperated on MN.
This type of comment is exactly why.
My DSD was in no way "pushed out" by either half sibling (my son or her Mum's daughter).
She was and remains a very much loved, included and integral member of the family.
There is nothing in the OP's posts that remotely suggests she (or her DP) intends to alienate the SC as a result of having a baby.
Of course the children need to be treated with sensitivity about this news but there is not reason it should be traumatic.
DSD was thrilled by the arrival of her half siblings and is close to them both.
OP it's understandable you are upset.
Her reaction was utterly inappropriate.
You have the choice to let it go and as a reason to distance yourself from her and focus on the support and love from your family or you can tackle it head on.
Personally I'd do the latter and send a letter/email from you and your DP to explain how much they have hurt you and frankly how inappropriate their reaction was.
You can then decide next steps depending on how they respond, having had the chance to get over the shock of the news and reflect on the situation.