I have a lot of sympathy for you, alycia. I have a friend who's been going through the same thing with her own daughter, except she both her children are legally adults so her GD is the only small child (her DS has ASD though and she also looks after her own mother - GD's great-GM - in the same house, so she's still pretty busy). GD is 5 and best friends with my DD3. The daughter had GD when she was 15 and promptly dumped the poor thing on my friend and scarpered off to Ireland for over a year with the then-BF. My friend has effectively raised her GD as her own, with the mother breezing in and out of GD's life whenever she feels like it, then disappearing again when she's back on the drugs and taken up with yet another man.
She got pregnant again, and my friend just had to call SS because the daughter was back on the drugs and quite obviously couldn't take care of herself, let alone another baby - and my friend just can't take on another child. SS tried to work with the daughter to get her into those young-mums-&-babies units where they help them get off drugs & teach them how to be able to care for their kids themselves, which frankly would be the makings of the girl and put her straight - but she refused to co-operate.
The baby - a little boy - was born 2 months premature and was finally able to come home from hospital just before Christmas. He went straight into care from hospital.
My friend really did do everything she could for her daughter, but at the end of the day her GD had to take priority - her daughter is legally an adult and if she wasn't going to help herself, there wasn't really anything my friend could do; but her GD needs her far more. She's provided a stable, loving home environment for her GD for the past 5 years. It's not my friend's fault her daughter went completely off the rails; some kids sadly just do reach teen years, fall in with the wrong crowd and go bad. My friend did her best, but she has to concentrate on GD. It broke her heart that she couldn't take in her GS too, but she's not a young woman any more.
So I do understand what you're going through and how hard it is. I've seen what my friend's gone through. You will be doing the best possible thing for both your GS and the unborn baby by putting your foot down and insisting you will not take the baby on. Your first duty now is to your GS and your other two young children; your daughter is old enough to fend for herself. It sounds harsh, but frankly she's made her own bed and now she must lie in it.