Sick chicken; any advice?

We've been raising chickens for almost 5 years now, many have came and left, and we lost most of our original hatchlings except one. Her name is Chloe, she's a 4 year old Barred Rock, almost 5, and is dying as of this moment and I'm afraid it's too late. Chloe grew up with another Barred Rock and those two were inseparable; they were basically one chicken. Roughly a year ago, Chloe's sister died of rat poisoning. Progressively, Chloe's behavior became more mellow like how a really old animal acts- except shes only 4. She stopped caring for herself and recently we had to bathe her since she got careless with how she'd poop and it created build up. She stopped using perches (in fact would fall if I tried to help her use one), stood or laid in corners by herself more, and I noticed she became much lighter; mind you, she's a large bird, you'd expect her to be heavy- and she used to be.
About 3 or 4 days ago, my brother went to clean the chicken coop and found her laying there. He tried moving her and she wouldn't stand. He took her out and I tried feeding her bread but I think she was too weak to eat it or something. At first, she'd peck at food here and there, but now she flat out won't eat even the softer foods I've tried giving her. Now I can't get her to eat, and I cant get her to drink at all. She wont stand anymore, now all she does is lay there and sleep, pooping on herself. Yesterday I went to clean her off and noticed she had no meat on her at all. She's almost all bones now.
We don't know whats wrong, we aren't sure if shes just been rolling downhill ever since her sister died and shes just so depressed she's given up, or if shes sick, we just don't know. And my family doesn't see the worth in taking a chicken to the vet, but to me they're still our pets and I can't stand watching her lay there and die anymore.If ANYONE knows whats wrong or knows any ways to help, please tell.

Welcome to BYC. I'm sorry about your hen. She could be suffering from so many different problems, but it sounds as though she may be suffering. If she doesn't respond to giving her some water with vitamins and electrolytes, some scrambled egg, and getting her warm, then I would consider putting her out of her misery. There are reproductive problems that can cause illness in hens her age, and she also could have any number of illnesses. I believe in making them comfortable, but not allowing them to suffer.