English Observation: How Many Siblings Do You Have?

I used to work in retail, and the staff always comes and goes, I remember this conversation that the two other staff Jerry and Nick had a few years back which I found pretty funny at the time.

Nick: "Hey Jerry, How many siblings do you have?"
Jerry: "I have no siblings, only one older brother."

It took a few seconds for me to go wait… what? So I turned to Jerry and asked him "You have one older brother, that means you have one sibling?"
"No, sibling means younger brother or sister...", he replied with such confidence.

I looked over at the other staff in the store with a confused look on my face, I was pretty sure the definition of the word sibling includes older or younger brother or sisters, but the confidence of Jerry not only made me question myself, all the other staff in the store were starting to question the definition of the word as well.

So we ended up Googling for the definition of the word and confirmed that it does include older siblings as well.

I just find it very interesting that the confidence of Jerry, could make all of us question a word that we heard a million times and use on a regular bases.