So we had two cats, until recently when we had to put one of them to sleep. They are from the same litter.
Sweep was always more whiney, especially around food time, whereas Sooty was a bit more laid back, but would also Meow incessantly if they wanted food. (A trait I have always found annoyed the living bejesus out of me).
Sweep is not in a good way right now, meowing day and night and only cuddles will help, but that doesn't help when we sleep, and we can't hold him 24/7. He is fit and well as best we can tell, has a good appetite. He has always been very highly strung.
I have heard animals sometimes can't get over the loss of a sibling and need to be put to sleep as well, which given we have lost a Dog and a Cat in the past 4 weeks, I am very keen to avoid. Having said that the Meowing is exceptionally annoying and driving us all insane.
I am unsure if it will resolve, but I am thinking it may not.