My situation was/is an adult stepdaughter with young adult children. Stepdaughter on benefits due to ill health, middle child had a breakdown, was sectioned for 48 hours and then moved home. Every time keep was mentioned he started becoming overly stressed, so a good few times I did a online shop for them. After 18 months of this middle child was becoming rather over weight while dsd weighed about 7 stone. At that point I made it clear to dsd that while I would always be there for her, my help had to stop somewhere, so if she had no food, call me and she could come and stay until the situation improved, as my freezer is always full.
Since then, middle child has passed his driving test and nows runs a car, my refusal to help has forced a situation where stepdaughter is facing up to the fact that even mentally unwell people have to pay their way, she is slowly starting to enforce firm boundaries with her son. For various reasons dsd learnt from her childhood that SHE was responsible for everything with no right to her own boundaries, financial or otherwise. Dsd is experiencing a crash course in parenting adult children and its working.
Its really hard to say no when someone is hungry, so explain to your mum that you can not financially help her any more, ensure your siblings are saying the same (and sticking to it) invite her round for dinner if she has no food, or invite her to stay over.
Make sure dm has contact details for suitable organisations to help her, then stand back. If she asks for money, "im short myself this month, but I can feed you, so come round for dinner" is a great answer. Then drop the rope. Your dm is an adult and gets to choose for herself. If she makes bad choices, the she needs to experience the consequences of that, else you and your siblings will be supporting her for the next 30 years or more.