Hi. I had two brothers from the same litter/same age that were inseparable. They were glued together... Stanley passed away at age 16 from kidney failure and complications, Pookey at age 19 from a brain tumor.

The last week I had Stanley, Pookey hid out and I rarely saw him. Their relationship totally changed. Whether it was the smells of the vet or he knew how sick Stanley was. There was no hissing, but there was a complete personality change.

I am sure your cat know something is wrong -- both the sick one and the healthy one -- and it is affecting how they get along.

It took Pookey close to a year to get his balance back after losing Stanley. He was in bad shape (meowing constantly and searching) for about 6 months, and then slowly got better until he was in new routines at about a year...
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Tammy
& Sparkle
(Stanley 8/91 - 8/07)
(Pookey 8/91 - 11/10)