*hugs*
I know it must be really hard for you living with this and I really do hope your mum can keep on the right track.
One thing is important though - she's really got to want to stop/cut down/get help. It won't work otherwise, same as with other problems. I know it sounds really harsh but sometimes it takes losing people through drinking for somebody to really try and get better. It might be a good idea for your sister to keep your nephew away so your mum can see just how serious this is, because that's really what she needs. I hope that makes sense.
I'm not sure if
this service is in your are but check it out. I'm currently using the services and am happy to answer any questions about it or anything else you might want to ask.
Stay strong lovely <3