awww hugs hun
the last six months before they go to school is the hardest time. I was lucky i was warned about this time from a friend or I would be at a loss as well.
This boundary pushing is how they try and work out how they fit into the world and their role.
If found that the busier I kept the twins around this time- the less challenging their behavior was. Its the mental stimulation and the curiouristy to see what would happen if they did certain things that needs to be fed- aka the WHY time!!!
hang in there- it will pass but having pre-set consequences and defined boundaries really does help. In other words- a list of consequences that she knows will be put into place if she does a behaviour that is unacceptable. Also saves having to think up consequences on the spot
Also try and get her into some apps that may challenge her such as Reading eggs and Mathletics- use them as a reward for when she is doing what is deemed 'good behaviour' and hopefully the mental stimulation may help kerb the challenging behaviour!!!
hang in there - you have entered the WHY stage!!!