First of all make sure her teeth are ok, if she wears dentures make sure she is A, wearing them and B they are clean.
Get your Sister to make sure her tongue isn't coated , this can be a sign of oral thrush and makes everything taste funny. You can get Nystatin from the chemist for this.
Then encourage snacking rather than big meals, until her appetite comes back, everything full fat, milky drinks, cheese & Biscuits with some grapes at the side, yoghurt, especially the ones with added protein, or ones for Gut health. Probiotic drinks . Porridge sachets of even the pots , some nice soft rolls with Egg and tomatoes, or tuna , if you can up her protein her appetite will improve. She will probably eat better with a bit of company. I realise that can be awkward when you are not living near by.