After the 1979 one-child policy (独生子女政策), most urban 80s and 90s kids have no actual 兄弟姐妹, so the word took on an aspirational feel. The policy was relaxed in 2016 and fully ended in 2021.
usage
A four-character summary compound: 兄 (older brother) 弟 (younger brother) 姐 (older sister) 妹 (younger sister). Chinese distinguishes sibling order lexically; English collapses to 'sibling.'
