Paying for alcohol detox

Until her daughter is serious about wanting to stop then I fear your sister is wasting her money & £30k sounds a very expensive detox
If her daughter does become ready to stop then she should sort it out herself.
I stopped drinking 20 years ago so I do have experience but I never did a detox I just stopped with the aid of AA all for free. But this again depends on the daughter I knew i could stop safely cold turkey as I was a binge drinker and very often went long tines without alcohol

It sounds as though she isn’t ready to stop drinking. She’s also blaming others for failed detox.
That’s a huge amount to invest in these circumstances

It's a big risk to take especially as the neice had already started a couple of detoxes, which weren't followed through completely. Sadly some alcoholics aren't fully committed to stay off the drink as there's always an easy excuse.

I think that she is not ready to stop drinking , if that was her priority she would have stayed in detox.

A drinker that wants to continue drinking will always have a reason/excuse that they cannot embrace help and support. Trying to do it to please others will not work.

Knowing that her mother has put herself in debt will likely make her become resentful towards her mother when she fails again.

It is a very sad situation but personally I would advise not to do it.