Gosh, a prodigal sister rather than a prodigal son!
I would advise your sister to halve her saving right now and put this into a bank account no one, especially her son, knows about.
Hoarding and compulsive shopping are anxiety issues, so counselling for that may help.
You could explain you are unable to help but offer to assist in the sale of hoarded and surplus goods to boost the coffers, if you think she'd part with them.
I would suggest she tries toughlove with her son but I cannot see that happening tbh.
I have a prodigal sister as well and she bled my mother of thousands - simply because Mum let her. After Mum died, she started on me but I'm in no position to help anyone financially, so all her pleadings and threats fell on deaf ears. She simply had to shift for herself.
You may have to risk the permanent rift if your sister does go broke and is desperate to help her chick.
She could even be pushed into taking a loan from some shark.
But if you normally have a caring relationship, just telling her your money is tied up, should not lead to a break.