extended hours

my store is now doing holiday hours (8am-10pm) all year round and corporate hasn't allotted anymore avaliable hours. me and my SM got into it because I (21F) don't feel safe if I'd have to work that closing shift without a cashier. My SM always covers and has enough people for the shifts she works but never considers others shifts or accommodations for her days off. Am I the unreasonable one or is the truly an issue?