Complex sentence: The summer before they went to Middle School, Kate went to a camp of some sort, and when she came back, she got a bra and became Lizzie, Gordo and Miranda's enemy.
Split into simpler sentences: The summer before they went to Middle School, Kate went to a camp of some sort, and when she came back, she got a bra. From then on, Kate became Lizzie, Gordo and Miranda's enemy.