Hi ClaireH,
Welcome to the Forum.
Your story is so sad and like you I would be very concerned for your Niece, it’s not a good environment for her to be bought up in, can you try and talk to your Niece to see exactly what goes on with her Mum?
Although it is very kind of your Mum and Dad to be helping your Sister, really they are enabling her, she must have money to buy the alcohol.
I think I would be doing some serious talking to your family about the concerns for your Niece.
Does your Sister have any friends that you know that you could talk too that perhaps live nearer her?
If you are really concerned for your Niece perhaps you would have to contact Social Services.
The Icarus Trust is a place where you can get in touch with with your concerns, I think you can email them.
Keep in touch on here, take care.
Dx