Hi OP. Sorry to hear you/your sis are going through this.
How old is your nephew?
I’ll take a leap and guess he’s definitely an alcoholic if things are getting this desperate. Ultimately, there is absolutely nothing you can do To make him get better if he doesn’t want help and assuming he’s an adult. I know that’s horrible to hear but it is the first, second and last thing you need to understand, even though it’s a horrendous thought.
The first thing is to protect your/your sister’s own sanity. If you haven’t already, Get educated on the problem, particularly around enabling and codependency. There are some really important pitfalls you need to avoid. Alanon would be worth considering (sister org to Alcoholics Anonymous and supports families). Your sis may have to make some really tough decisions for a mother if she wants him to get to a point of wanting help (e.g. cutting off any financial help, asking him to leave her house).
It sounds as though you have already tried talking to him and it hasn’t gone well. I’m afraid the NHS addiction services are massively underfunded and the best help is private, if you have the funds. He would still have to accept help, though. You could try an intervention. Ideally, you’d want to be really well organised for this and some of the private rehab centres can help with this, if you have the means. Private funded help would include outpatient or inpatient rehab. For NHS, you’d need to get him to your GP as a starter.
It’s a hard road and he’s unlikely to stick with it unless he truly wants it.
Good luck!