I caught my brother (twice already) listening to my sister have sex with her boyfriend. Is this normal?

Anonymous
When my parents are not around and everyone is out or sleeping (or so my sister thinks) my 20 year old sister has sex with her 28 year old boyfriend at the house. On two separate occasions when I come from school I have caught my brother outside of their bedroom door listening to them have sex. He is making the habit of coming out early from his job just in hopes of catching my sister and her boyfriend have sex.
I caught my brother (twice already) listening to my sister have sex with her boyfriend. Is this normal?
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