Complex sentence: Katie was already angry with Perdy over the surrogacy and the way Perdy was now treating her and her revenge was to let Perdy think she was unhappy and spent more time at Home Farm.
Split into simpler sentences: Katie was already angry with Perdy about the surrogacy and the way Perdy was treating her. Her revenge was to let Perdy think she was unhappy and spent more time at Home Farm.