Why so complicated?! And also similar to Wild Things?

The movie was ok, but they complicated things way too much! I couldn't get my head around the last 25 minutes or so of the movie.

What was even the point in Alex in the sister's plan? Surely Nina could have found a way to kill off her new husband and take his money without all this complication.

Also I thought this was extremely similar to the movie 'Wild Things' starring Denise Richards and Neve Campbell.