sis
n. countable B1 Intermediate US //ˈsɪs// UK //sˈɪs// sis
n. a short, friendly way to say 'sister'. You can use it for your actual sister or for a close female friend.
n. a shortened form of 'sister', used as a term of endearment or a familiar address. Common in informal speech and digital communication among peers.
Hey sis, are you coming to dinner tonight?
She has been my best friend since primary school, so I usually just call her sis.
Usage
Often used as a vocative to address someone directly.