My brother found out I cut myself.

I have 6 brothers, five older and one younger. usually I'm more careful about the cuts but I was playing xbox today when I rolled my sleeve up(kinda in the moment) I felt my little brother(14) touching my arm and just though he was trying to ask me something. When I looked down I realized he had saw the cuts! He asked me what happen but I told him I scraped my arm while skateboarding. Next thing I know he's in my room and when I ask him what he is doing, he says he is looking for what I used to hurt myself and if he didn't find it, he would tell my older brothers! I would usually expect my older brothers to do something like this but not my younger brother. What can I do to stop him from telling my brothers, and why is he like this when he is my younger brother?
My brother found out I cut myself.
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