Why do I feel uncomfortable around my shirtless brother?

I'm not attracted to my own brother but I feel uncomfortable when he's shirtless. I'm terrified at staring at his chest or something.
When we're eating at the table I can't talk to him because his lack of shirt is distracting

He gets annoyed when I tell him to put a shirt on especially since it's hot indoors.

We're not close and we barely hug each other, I don't know if this information helps
Why do I feel uncomfortable around my shirtless brother?
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