So that expresses the PURPOSE
My sister has taken a job after school so that she can buy a car.

Because shows the REASON.
My sister has taken a job because she has no money. (Lack of money is the reason for taking a job).

NOTE:
The so that clause often contains: can, could, might, would or will. It comes after the main clause.

Dave is driving fast so that he will arrive on time.

Nora eats green vegetables so that she can get enough vitamins.

Sarah wore a pink dress so that Anne would recognise her at the airport.