If the situation is bad, yes call somebody. It is sad to see parents drink like that. I would be careful if I were you, being that you are 18 and she is only 16. She is underage and you could get in trouble if things go too far. I mean with your relationship. Sexual wise. If you two aren't, that is great. Safer for you.
I really don't know what to tell you. Other than what if you talked to a counselor. Somebody professional and they could give you some good solid advice. I know that you don't want to see the children split up. And if they get taken away, that may happen. But are they living in fear right now? Is the conditions unsafe? If there was a gun involved, that is really dangerous. Even if it wasn't loaded. The mother sounds quite unstable. But the boyfriend doesn't sound as bad. Can you talk to him?
I hope that you can get some help in deciding what to do. It really sounds like you should talk to somebody who knows the system and who can give you some better advice than I can. But keep posting. I will give you some teen sites too to post on if you feel like it.
http://nineline.org/
http://forums.teenhelp.org/
http://www.teenhut.net/
I get the feeling that the thing holding you back from calling somebody is the thought that you might get seperated from your girlfriend. I hope that isn't standing in the way of your decision. Do what is best for her and her siblings. Keep them safe.
Hugs, Karen