How we cite our quotes:
Quite where I had heard the voice before I did not know. I knew only that these two words sent a tremor of joy and hope and expectation through my body. (17.4)
Just as children never forget the sound of their parents' voices, the sound of Albert's voice stays ingrained in Joey's subconscious.
I think perhaps they cared for both of us as if we were their brothers. (18.24)
Even though Albert's parents never quite got it, the other soldiers see how strong the bond is between him and Joey. Maybe it's because they've formed similar bonds themselves, while Albert's parents always looked at horses as farm animals.
"[David] looked after me, Joey. Like a brother he was to me." (19.5)
Don't worry, Albert can also form familial bonds with people (not just horses). It's unsurprising that David and Albert became so close; they're both only children without actual brothers or sisters of their own.