saviour sibling
noun [ C ] UK (US savior sibling) uk/ˈseɪ.vjə ˌsɪb.lɪŋ/ us/ˈseɪ.vjɚ ˌsɪb.lɪŋ/
兄姐救星（生来就有某些特殊基因的孩子，这些基因是经过特别选择的，用于治疗孩子罹患绝症的哥哥或姐姐）
a child who is born with particular genes that have been chosen in order to treat an older brother or sister who has a disease