also: きょうだい
older sister and younger brother
Refers specifically to a sibling pair consisting of an older sister and a younger brother. Not used for other sibling combinations.
They are an older sister and younger brother; the sister is a university student and the brother is a high school student.
The older sister and younger brother went shopping together.
Standard kanji spelling for the reading してい, specifically meaning an older sister and younger brother pair.
Gikun reading; the kanji 姉弟 is normally read してい. Reading it as きょうだい is a special, non-standard reading used in some contexts to emphasize the sibling relationship, but it is uncommon.
Compound of 姉 (older sister) and 弟 (younger brother). The reading してい is the standard on'yomi combination. The gikun reading きょうだい is a special reading applied to this kanji compound to convey the meaning of siblings, but it is not the standard reading.