여동생 is short for 여자동생. In the same way, you can call your younger brother 남동생 (short for 남자동생). Unlike older brother or older sister, these words are not gender-specific. 오빠 older brother to a female 형 older brother to a male 언니 older sister to a female 누나 older sister to a male

My older brother almost always calls me just '야' plainly. Other brothers would call their younger sister her name.