Hello Shelagh
I am also from Scotland... Dunoon, on the west coast.
I am so sorry you are having to go through this, please dont hesitate to private message me if you feel like it.
I am sure your sister has access to nutritional suplements on the NHS.. but if she has lost heart you are right you cannot force her. Maybe she would like a milk shake with lots of ice cream and a carnation instant breakfast in it...
I would suggest that she ask her Doctor for an anti-depressant, it cannot hurt.
I wish I knew what to say to make you feel better , I know you are really scared for your sister. Don`t hesitate to ask if there is anything we can do to help.

Take Care
Marica