step-siblingnoun [ C ]
mainly UK also mainly UK step sibling, US usually stepsibling uk/ˈstepˌsɪb.lɪŋ//ˈstepˌsɪb.lɪŋ/a child of your stepfather or stepmother with another person who is not your parent:
Step-siblings have no parents in common, whereas half-siblings share one parent.