Try to show her that drinking is not going to help her to grieve the losses. Just like she's finding it difficult to deal with the loss of her parents, you're also facing a difficult time because of the way she's reacting to the loss by putting herself in harm's way. Be gentle with her. Convince her that her current state is making you become worried about her well being.
First, you shouldn't feel that as a burden. Like if you don't manage to do it, to feel guilty. Your mom is a grown up person. Yes you can help her. And yes, you might be the one in a best position to do so. And yes you should try, but you shouldn't feel this burden of guilt. Your mom is taking the conscience decision to do what she does. She will be the one who will take it back.<br />
Of course you can help her. She might be drinking because of some inner loneliness. She lost her mother( your grandmother) she loses you ( you get married) and she is estranged to her husband.<br />
Its not, that she is drinking. Its about why she does it. I'm not proud that I had some getting very drunk occurrence, and always did it when I was really down. So I would guess she has another problem she tries to forget. <br />
But speak with her. Be with her more often. She might feel abandoned by everyone, since you are getting into a new family now. But being close to her, might work better than you thought.