The word "sis" is short for "sister" and is often used to refer to a female sibling or someone close to you who is like a sister. It is a friendly and informal way to address or talk about someone. Full definition
Hy Peskin, 89 Most of the golf photographs that appeared in SI in the 1950s were taken by Peskin, who died on June 2 of kidney disease. (si.com)
Congrats to Roger Goodell and Shad Khan on making the cover of SI for pretending to care about racial injustice enough to make more money (sbnation.com)