And you have to get glasses. Much research has shown that a simple pair of glasses is the most effective disguise in the world.
A low INT or CHA might mean that your character would do a very poor job of a double-life, though, as she would be hard-pressed to accurately portray a different person.. which in turn would be part of your own RP, as you'd play her poorly playing her alter-ego.
However, as the others said, if Sonya can pull it off (whatever her stats are), she might be well-served by taking on a different dialect... speaking a different language, if she has it... using different spells on a regular basis, dressing differently, etc. She would also be best off never summoning her familiar in her alter-ego, as that would be eminently recognizable. And unless her alter-ego has some reason to hate Sonya, the best reaction to somone calling her "Sonya" would either be a blank look or a tell informing them that you don't, in fact, look like Sonya.. or asking them to make a Spot check to see if they do see through the disguise.