OP, i really feel for you here. And for your Dsd, and your Dw. But unlike your Dsd you can’t make a decision and unlike your DW you can’t influence a decision.
But you have your own compassion and care for your Dsd, as well as (presumably) love for your DW.
However much I loved my new grandchild my heart would sink if one of my Dc had a baby at such a young age and before they were able to be independent, and at the thought of endless night wakings, washing machine and wet washing everywhere, just general baby life.
But you are where you are.
If it was one of your own kids who was pg and planning to go ahead, how would you expect your DW to engage? What would you hope for in terms of support, and which areas would you feel torn between supporting your own Dc and knowing the impact on your DW?
Unless you do live in a vast palace with great sound proofing, the nighttime waking is likely to be an issue for the whole household. Unless you can all sleep through the disturbance, some people can, I suppose. Will this affect other teens doing exams?
Can you suggest a room move-around that puts your Dsd in a more self contained situation, e.g in the loft conversion if you have one?
Do all the children have quiet study areas so that they can continue to do well at school despite the extra activity (especially if Dsd stays with you until your DGC is a toddler!)
Would DW changing her hours affect your family finances, putting extra pressure on you? Personally I think beyond taking the equivalent of parental leave, to give her Dd the support she would have had from the Dad, it does risk infantilising getting Dd to start compromising her own life and work to become a part time (grand) parent.
So I think all you can do is try and be supportive, helpful and constructive in finding solutions to the practical issues (as you are being), while being optimistic about how in the end we do all fall for a baby.
But also find ways to talk frankly but without criticism to your DW about your own boundaries and feelings.
Not easy, but I wish you all the luck in the world.