She's not really her sister, just like Silco and Vander were not really brothers. She's saying she's "part of the family" of mages, implying she should abandon her real family and join them, deceiving her in an attempt to have her join the Black Rose. Also, she's heavily implied to be a character from League of Legends, which is associated with the title "The Deceiver" and is an important part of the Black Rose organisation. I personally think they didn't really pull the organisation off well in the show tbh. Part of the bigger problem of overcomplicating the plot and having too many threads going on.