sounds like Jo found an effective way to express her jealousy...

When I was younger I had a dog, Candy, who was more like my sister cause my mom was "pack leader". When Candy got old and was obviously not going to be around much longer I was the one in the family to delay putting her to sleep. I'm sure she wasn't feeling great for a while - she couldn't run and play anymore, but she could still walk slow and she could still look at me with life in her eyes. The day her eyes lacked something - happiness? life? - was the day I agreed it was time. It was as though she couldn't notice things that made her happy around her because she was concentrating on breathing.

I just believe the dog's appearance and actions will give you the signs you need to decide.

By the way, Candy was never put to sleep, she died in my arms on the way to the vet. I don't think she was ever in "too much" pain, and I'm glad I held off my decision.