So sorry Bob. I know it is incredibly hard.
I had 3 cats all from the same litter. One lived to about 18 and the other two died within days of each other just shy of 20.
One of them just faded away in my lap and the other demanded to be let out (she spent nearly 20 years as a house cat and she suddenly was desperate to go out in the middle of the night in the rain). It was difficult but I decided she was in so much distress I had to let her go. She marched off into the night never to return.
I loved them all so much it still brings tears to my eyes, but cats know when it is time to go.
She will tell you. She looks comfortable there on the sofa.