Well, it does sound a bit fanciful to me, though the fact that "cissy" is the British spelling gives it slightly more credence there.
Still, "sister" seems the most likely from a semantic point of view. Sister - woman - weak - cissy
Narcissus was defined by his egocentrism rather than cowardliness or weakness.
I felt the letter authored by Lt. Mark "?" evidenced a narcissistic personality and cowardice, in no particular order.
Hence my comment cissy and sissy were "interchangeable"
I love semantics;)
sp