be gentle, be docile

My younger brother is gentle.

私の弟はおとなしいです

My younger brother is gentle at home.

私の弟は家ではおとなしいです

My younger brother is usually gentle at home.

私の弟は家では普段おとなしいです

My younger brother is usually gentle at home even with people he doesn't know.

私の弟は家では普段知らない人にもおとなしいです

My younger brother has a gentle nature at home even with people he doesn't know.

私の弟は家では普段知らない人にも性格がおとなしいです