Im sorry to hear about your loss of Sara. I would notify the vet of Charlie's trouble. It sounds like it could of been a seizure to me. it would not show up on blood tests. There are different types of seizures dogs can have. If he has them repeatedly the vet may put him on medication. Some breeds are more prone to them then others.
Sorry to hear about Sara. Definately keep an eye on Charlie. I had a lab that if he would get over stimulated or run to hard he would wander (stagger)around the yard, all of his senses shut down--it was like he couldn't hear me. He basically looked like he was stoned. Ocassionally he would collapse and drool uncontrollably, shake, pant and have that dazed look in his eye. Depending on what kind of dog you have, he may be prone to certain diseases. Like you, my Lab lost his "sister" of 8 yrs that he grew up. He went through a horrible depression, and his health took a nose dive after she died. All of these different problems just started popping up or getting worse after the death of his sister.
I would definately call your vet and explain what happened. Better safe than sorry.
Good Luck
Sorry to read of your loss and the loss of a buddy to your remaining dog - this is one of the issues that troubles me when we talk about getting our dog a "brother or sister".
The one experience I have with a stroke, diagnosed as a spinal stroke, was there was remaining paralysis. In this case it was a 10 year old large Golden Retriever. We got him back on his feet (rear was the problem and is a weak spot anyway in Goldens) using water therapy. My conclusion from this experience is there will be long term affects, if the dog returns to "normal" I'll guess it was not a stroke. Or if it was it was a "mini stroke" and may make little difference unless it suggests some treatment to lower blood coagulation - e.g., coumadin in humans, or maybe just aspirin. I do not have any experience on what is done in dogs.