[matese]

Speaking from my own personal experience, I had a pack of 3 female yorkies all 3 years apart, the baby was not really close to the two older ones but she preferred me over them, the older girls treated her as the baby always washing her face. I lost the oldest one then 6 months later I lost the other older girl. The baby who was now 6 years old had been with the pack 8 years, although she was not close to them she went into deep grief, her entire personality changed, she was my little clown now she was a recluse. I was very surprised that she took the loss of the 2nd girl so hard because she was never that close to either of her older sisters.

I gave her 6 months to adjust to being the only child, that never happened so off I went to find her a BFF. Now this little girl was use to being babied by the two older girls and she was a bossy spoiled lil diva, I didn't know how she would handle being the momma when I bought a puppy. So I found the perfect buddy for Ms bossy pants, this was a much bigger yorkie. Ms bossy weighed 9 pounds, the puppy at 8 weeks old was much bigger. I bought her home, set her on the floor, Ms bossy pants ran up to the puppy as if she was a long lost friend. It was love at first sight and I had my lil clown back. These two girls became inseparable and were BFF.
I tell you this story because although Cookie never showed a bond with your Gigi she was in his life for 5 years and yes, he misses her, but that should be no reason he would be nasty with you when he wants belly rubs or just petting.

I now have a rescue boy, he was surrendered, not abused but was spoiled rotten. He was 2 1/2 years old when I got him, we're a team 5 years now. I know about his background, but I am still learning about him as he is still learning about me. I don't know how much you know about Cookies past, you did mention if he got stressed he would he would direct it towards Gigi so you know there is a issue with him. Be aware of what triggers this behavior and if possible try to avoid situations that brings out the nasty behavior. I know what will trigger my kid to want to snap at me although he knows snapping comes with a sharp reprimand and no attention from me, I avoid these triggers because I do not like to speak sharply to him.
These are my experiences with the loss of a pack member and a rescue.
A vet check is good to make sure he is in good health. These dogs are good at masking pain.
Please let us know how things are after his vet visit.