弟弟 refers to a younger brother.
Topic 1.1: 1.1 Families and Communities in China
弟弟 (dìdì) cheat sheet for homework
姐姐 (jiějie): Refers to an older sister.
兄弟姐妹 (xiōng dì jiě mèi): Refers to siblings.
哥哥 (gēge): Refers to an older brother.