You sound genuinely worried about your sister ,I wouldn't over react though , I am presuming your mam goes to work and cooks and cleans for you all , maybe a bit of help is needed ,pitching in as a family ,chatting to her about the situation instead of pointing out what she should and shouldn't be doing about it ,actually talking to your sister about the worry and affect her behavior is having on you all ,instead of confrontation all the time ,
Keep communicating ,let her know she is loved whatever ,offer support and guidance ,and be there for her if it all goes tits up ,
Don't give up ,and