Is my cat dying? Plus will my other cat be able to indicate to us she is?

My cat is about 15 years old. Maybe 14. She hasn't been eating much lately. She's laying in the bathtub right now. It's 3am. She wont eat her treats.. which she usually goes crazy over. She has flea's every time this year. and she does right now. She is the apitimy of evil.. lol. She lately has been laying by me lately.
We put flea stuff on her (frontline) because last time we used walmart brand she got sick. both of my cats did. We also sprayed the floors with flea spray. Also she wont even eat lunchmeat? She lovesssss human food. we have been trying to get her to eat anything. Like she still begs for it. But won't eat it.?
She had blood in her poop a few days ago. Now she has diariah sp?
Is she just sick? Should I get ready for the worst?
I have another cat also. He's been part of this family for a year now. Would he be able to tell if she's dying? Like would he lay with her more or something to indicate she's dying?