I like
Norah;
Christina is better than
Christine but not as startling as
Christiana; and
Sarah doesn't appeal to me. In a world that contains the name "
Sarah", I can see no need for sleeping pills.
There used to be little paper dolls in a magazine that kids could cut out etc - their names were
Norah and
Tilly, so keeping your -a theme, what about
Norah and
Matilda?
With
Constance and
Prudence,
Charlotte is a bit out of step.
Clemency or
Clemence, perhaps?