In reply to mypyrex:
Sorry to hear that. Does she live alone? Have you noticed any signs of confusion? Memory loss? A tendency to repeat things she's already told you? My step mother started exhibiting these signs at the same time as losing weight when she was in her late 70's. She also started to become slack about personal hygeine, which was surprising as she had always been fastidious and was a smart dresser.
All of these were signs that worried us and when we eventually got her doctor to get a referral to the hospital for tests they became concerned about her heart, and generally weak state. She also got a series of urinary infections which can result in confusion. Because of her heart condition she was admitted into hospital where she remained until she died from a heart attack 4 months later. During her decline which was startling and rapid she continued to lose weight, just picked at her food (she was put on a drip, but tended to pull it out) and gave up trying to walk. She became agressive towards family, nursing staff and friends. Her decline into what appeared to be dementia was also startlingly rapid, but even this was eratic, some days she would be lucid and fine, the next she wouldn't recognise us.
I do hope that you can get help for your sister in law and get her eating, because she must build up her strength, or she may get into a slippery slide like my step mum did, and that all started with her going off her food.
Good luck.