
I wouldn't write off the connection of this with the arrival of your son as quickly as that. Has her routine been changed at all since the arrival of the baby? Is she getting the same amount of attention? Is she getting the same amount of exercise, by the same people?She's had two major disruptions in her life (the arrival of your daughter, and now your son) within a short space of time (you say she's only two) - I'd say she's anxious and feels pushed out ... and by being naughty, she gets attention, even if it's negative ...
I'm sure you've tried giving her loads of one-to-one attention, and praise, when she's being good, and stuck with the routine she was used to.