Do you know who I feel most sorry for? The OP. Not her DSD. Her dsd has obviously decided (like I did) that she is grown-up enough to have a baby. In which case, she MUST be treated like a grown-up, and MADE to understand the full implications of what effects her choice will have. On both her (dsd's) life, the OP's life, and the lives of her siblings.
I feel sorry for the OP as she is faced with what is basically amounting to a very stark choice - accept that she (OP) will be doing most of the childcare for a child that she has not chosen to have, and beggining to feel resentful of both her dsd and her DH for pushing this on her, thus probably irreparably damaging her marriage (thus also impacting on her other dc), OR to say that she will not do any of the childcare, thus ignoring her dh's wishes...thus he begins to feel resentful of her, and probably damaging her marriage.
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I fail to see what is wrong with a mother and baby unit, it was the best thing my mum ever did for me and my DD, making us move out, saying DD was MY responsibility.
Either way, this is easier for the OP's dsd to mentally cope with than it is for the OP. The OP's dsd holds all the choices in this situation, and by the looks of it, her dad (OP's DH) is so busy trying to 'make up for' his DD's crap mother, that his DD (OP's dsd) knows full well that he will go along with whatever SHE decides. Where are the OP's choices?
OP - please let us know how your conversation with her goes. And please, for now, get her to take folic acid - just as much as being over 35 comes with an increased risk of DS, so does being under 16yo at conception. V.V. important.
And it may be worth getting across to your DD that it is possible, no matter how young you are, to have a dc with SN. And if coping with an NT dc is difficult at 14/15/16, coping with an SN dc is a whole other ball game...
And to the person who thinks that it may be coercion into an abortion to tell your dc that they will have to leave and care for the dc themselves...errr, no! I knew that myself from the age of about 12yo. Didn't coerce me into having an abortion when I was 15 - I wanted to keep my DD THAT much. I didn't realise the reality of a baby, but I doubt it would have made any difference, (Bad childhood). I just factored into my decision the fact that I would not be living at home, and my mum would not be providing childcare, and it was up to me to deal with that. My DD has been told the same.