Why won't my brother clean his bathroom? Why doesn't it gross him out?

I have a restroom in my room. There is a common restroom that I never use. I could clean it but I want him to clean up after himself. I walked in there today after a month of not being in there & it's bad. It takes like 2 mins to clean the sink & toilet. Is he just waiting for me to do it? Any advice?
Why won't my brother clean his bathroom? Why doesn't it gross him out?
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