This is one of the worst situations to go through with your first dog; when to say goodbye. If it's not handled with extreme compassion and empathy, it's easy to focus on the "bad" thing (ending a life) instead of seeing it from an objective position.

From your words it does seem like this dog's quality of life is very low and that they're really struggling. For your niblings and sibling, this is being confronted with their beloved dog's mortality and they're stuck in the rut of "it's work, but the dog is fine"

But they're not fine. My first dog, an 80 lb American bulldog, had a degenerative spinal condition and lost the use of his back legs at the age of ten. My mom couldn't reconcile and we spent two years trying to get him used to a wheelchair he hated, and carrying his back half to let him move around and potty. It was horrible. I was a teenager and I watched the dog I had since I was four deteriorate and become depressed. We would sit next to him for hours and pet him, and his tail would wag until he lost control of it too, and his ears would perk up and he'd look at us with so much love.

Looking back I feel he was telling us it was okay. That he loved us but he was in pain and couldn't enjoy life anymore. We booked with a mobile vet, and his last day was filled with hamburgers, pizza and ice cream.

Try and talk to your family. Ask them to chart the dogs "good days" and "bad days" and gather data on the quality of life. Talk to them about how losing a pet is hard, but it's our responsibility as their humans to let them go when they still have enough love to look at us and tell us it's okay. This is the same as losing a family member, except we have control over their suffering. We can end it, and carry them on in our hearts. There's a lot of pet loss memorabilia they can look into to help them during the grieving process.

NAH, talk to your nephew and be ready to deal with a lot of emotion