Just don't know how to deal with my feelings about this

After a terrible childhood with a very unstable M, my GD was a heroin addict for 10 years and had her first DD (now 11) removed into the care of the FM who has made it impossible for my GD to see her. She successfully went into detox when her second child was born and has looked after her DS (now 5 )well. She has had a disastrous relationship with a man who boasted that he'd been detoxed 9 times and sees no need to work. I was relieved when she told me they'd split up. She's come to visit me this weekend and announced that she's pregnant by him. I am really struggling to feel pleased for her - the picture of her scan was the first thing she showed me as I met her and DGGS up at the station and she really wants my approval. But all I see is more expectation of financial support, an emotional burden on my GD that I don't think she is strong enough to cope with - a man who has no sense of responsibility and yet another child born into the benefits system.
My DS will no longer have anything to do with his D (I feel ashamed to say) and I have been the one stable and reasonably sane relationship in my GD's life. I did as much as I could when she was a child (though her M hated me) and have emotionally and financially supported her since she quit heroin. She is pretty hard going, few social skills, doesn't work and has no training in anything, doesn't have any interests but does dearly love her DS who is adorable. I am really struggling - it's the middle of the night and I just can't find a way to stop worrying about how to find a way to feel OK about the coming child.