Sis is a short, warm name for your sister, used when you speak to her or about her: “Hey, sis, wait for me.” It is family talk, very informal — the kind of word you say at home, in a message, or in a story with spoken voices, not in a school essay. Little brothers and sisters use it, and so do grown-ups who still talk that way with each other. Though actually, some young people also call a close female friend sis, even when they are not family. That wider use is slang, and not everyone likes it; the core meaning is still “my sister.”