I fell pregnant with DD when I was 15, had her just after I was 16. I too refused to listen to any talk of an abortion, as by that point, I had already made up my mind that it was akin to murder. (Not everybody's opinion, just saying, even young teens can have very strong views on those things). I was asked to leave my Mum's house, I went to live in a mother and baby unit. At the time, I hated my mum for 'abandoning' me. However, almost 13 years later, I can see why she did it. I had NO choice but to take responsibility for my decision to keep my baby, I had no choice but to get up in the night with DD, and I HAD to learn how to sho and care for a baby. The mother and baby unit was the best environment for me, they helped me to learn everything from simle first aid to budgeting.
I left school, granted, but was back at college quite quickly, using the on-site creche, and by the time DD was 15 months old, I was working. DD is now almost 13yo, and while I feel far too young to be dealing with teen tantrums, I feel proud of myself for doing so well with her. If you really are not happy with the thought of taking so much responsibility for your grandchild, then maybe it would be worth looking at mother and baby units, as it would save your sanity, and it would also force your DSD to be the responsible one when it comes to her child.
Having said that, I thought I was young! Maybe it might be worth putting it to her in such a way, ask your DSD what she would do when she is only 28/29 and her 14yo DD came to her telling her she was pregnant? See what she suggests then? Ask her how she would manage a situation when one of her brothers misbehaves...because her 'cute baby' will get older, and will misbehave.
The other thing is, seriously, leave any questions about the baby's father out of it for now, or your DSD will clam up and refuse to talk to you about any of it. It took me until my DD was 9 years old to tell my mum about DD's father. He was quite a lot older than me by 10 years, and it took me that long to realise he was weird for sleeping with a 15yo when he was 25. I just saw it as he was my boyfriend.
Good luck, whatever you do, but try and also see if there are some 'baby and bump' groups locally for her, aimed at teen mums. SS should have any details. OH and SS will HAVE to be informend anyway, because she is under 16yo.