Well unlike cannon Sunset the series implies she was a faithful student, but also that she was assumed killed with her allies by Sombra, add 1000 years. it's hardly surprising Celestia moved on or that she took another student. of course it's understandable that Sunset might not consider that and still feel betrayed. Also I'm going to guess she doesn't rush out and confront Celestia, and get all the information. Celestia explaining that she thought Sunset was dead, not imprisoned, and that 1000 years passed by.