Looks less derogative than Sissy, but it doesn’t change the fact that it’s an overly girlish name to give to a girl, especially a boy. It’s a dangerous name, no matter the spelling.
It wouldn't be a good idea to name your daughter Cissy, especially nowadays as "Cissy" is a slang term for "cisgender" (i.e. a person who is not transgender), and it's frequently used by the social justice crowd on Tumblr. Plus it only makes the name Sissy even worse with that association.
Sounds more like a nickname for Cecilia. I can't stand this immature, cutesy, yet ugly name that sounds like someone's calling a guy a sissy because they think the guy isn't masculine enough, like being feminine is the worst thing there is.