whois, he is crucial, and I really think EVERYONE parents better with support of others, your sister will need support OP.
I know the general advice is not to stick your beak in, but in this instance I would say your sister needs to know how precarious her situation is, (that she will be able to cope, and more importantly, will be allowed to keep the baby long term). She will need to access the services available and she will need to completely change her medications if she doesn't have live-in support.
TBH pregnancy is a great time for accessing extra support, the therapy I had been waiting for since my teens was suddenly available in pregnancy and shortly after. This is the time her life could really turn around. Before I had DC DH and I were party crazed animals - we drank ourselves into a stupor most weekends... as in we were wrecked from Friday to Sunday! I didn't know what was wrong with me then so I pretty much self medicated with alcohol and class A's! Somehow we managed to hold down decent jobs but i have no idea how.
But then we moved away from our home town and stopped going out for a few months and I fell pregnant with out first. We've never touched a drug since, I had a shot of caramel liquor in a hot chocolate at Xmas and I can't remember the last time I drank before then. I am addicted to my Vape though :D
I know the horror stories are more usual, but they aren't the only stories out there... I have no doubt quite a few who knew DH and me pre dc expected us to fall flat on our arse, but instead we got married brought a house and I had my last two DC whilst attending a full time degree at a RG Uni (we know how important RG is on here :D )
You need to talk to her. Help her. This could be the making of your sister and either way that baby will be your niece or nephew and they will need your help too.