Worried for sister who is damaging her health

Okay so, to make this clear from the start, I understand that she can do whatever she chooses in her life and I can only offer my help - and it's up to her if she takes it.

So I'm at university with my sister. Prior to university she said she would never drink alcohol given the health problems a family member had had as a result of drinking. At the start of university, she drank a sensible amount (for a student) alongside myself which I knew she would anyway. We went out no more than twice a week so we could manage our studying since uni was all new to us.

Upto christmas, it increased to around 3 times a week and she was staying up later and later and getting even more drunk. After january exams, it was refreshers week, so understandably, we went out a bit more but I expected it to cool down after that.
However, since then, she's been going out 4 times a week every week, getting stupidly drunk each time, and staying up until daylight consistently each night out. Now, to some this may sound like what some students do anyway. However, she's now increasingly missing lectures because of severe hangovers, she's spending excessive amounts of money each week to fund it, and has now ran out of money and is begging my parents for more.
In my mind, they shouldn't give her more because she's not using it for books or food, it just goes on alcohol and gives her the chance to buy more. She's frequently sick/ill as well and I know she'll be damaging her liver/kidneys by getting drunk 4 times a week since january.
I just don't know what to do to help her. I've warned her to cool it and limit her nights out but she just tells me it's none of my business. She's becoming so irresponsible and increasingly vain (despite her eye bags from lack of sleep) so she's getting further and further into the wrong crowds.
Her studies are suffering and I want her to be happy, healthy and successful, yet she just won't listen. Since we live away from home as well, my mum feels helpless especially since we're her only daughters.
I'm constantly 'on call' at night when she gets into a state and it's just not fair. As much as I just want to let her 'get on with it' and make her own choices, she's just on a downward spiral and it hurts to see her like this.
What can I possibly do to help? It's not helping that the people she goes out with are the same as her. But everytime myself or my mum try to help she just ignores us or calls us controlling. She's in denial that she's the way she is, and has a 'it won't happen to me' attitude. With exams at the start of May, I'm getting desperate to help her. She knows her family are there for her nomatter what, but she's taking it for granted. She appears to be the only one who isn't worried

Oh dear. You sound very reasonable and balanced in this post, and it's clear that you have her best interests at heart, rather than some need to interfere or make a drama. It's very tricky.

From what you write, it's clear that she's possibly not heading down the best path - and the drinking might be getting slightly out of hand. What's the relationship between the two of you like generally? Do you get on well - are you 'equals', or do you take on the 'big sis' role? Without knowing the dynamic between you, it's difficult to suggest a best way forward. For instance, if she resents you being a bit of a 'big sis', then she's going to be hypersensitive to anything which could be construed as a 'lecture'.

She's clearly pushing at the boundaries, and it's obvious that any form of disapproval is going to push her away and make it worse, rather than engage her.

Have you talked to her about it?
Why is she drinking?

It sounds like a hidden depression. How did she took the whole university thing?
You should talk about it all, just as best friends, trying to help.

Are you both competitive, like trying to be as good as the other or even better?
Does it seem she was trying to hard and gave up?