Opelle - well done for having that difficult conversation.
I am an alcoholic and fwiw I would have answered exactly as your Mum did: say it was a one off; minimise how much I drank at other times; compared whatever I was doing favourably with other situation - I agree with snorbs - typical alcoholic bullshit. However, I would have also reacted in a similar way if my dd questioned me on how much chocolate I had had (and I do not have a chocolate addiction) - part of it's a natural reaction to having one's behaviours questioned. What I am trying to say is that none of us can guess, from your Mum's behaviour whether she has a long term alcohol problem.
What she has done is behave in a way which was not acceptable for someone responsible for 2 children. And what you did (taking them home with you until the crisis was over) was the right thing to do.
If your Mum does not have a long term alcohol problem then your conversation with her may well have been the sharp shock that she needed to help her stop on a potentially destructive course. And if this is the case, then you (and others) can help her by providing some adult company when she is feeling lonely in the evenings, arranging a sitter so she can go to (non-drinking) group or club, meet up with her for a cup of tea, or a burger and coke. It will take a bit of effort but it will be realtively short term and will help her to adopt some better habits.
However if your Mum has a more ingrained alcohol problem, then it is likely that after a short spell of 'good behaviour' she will revert to drinking, and may be more secretive about it than before. (I do not think I would have given my dc a hard time for spilling the beans, but I would have worked out ways to drink without them being aware of it).
The key question is - how will you ever know the difference? Keep close, trust your instincts, encourage you dsis and dbro to trust their instincts and to be safe, reassure them that they are not grassing her up, spend as much time with her as you can, help her to get counselling support to deal with her emotions about the break-up.