collateral blood relative
Legal and genealogical term for blood relatives who are not in a direct line of descent, such as siblings, aunts, uncles, nieces, and nephews. Contrasts with 直系血族 (lineal blood relatives).
The Civil Code defines the scope of collateral blood relatives.
兄弟姉妹は傍系血族にあたる。
Siblings are considered collateral blood relatives.