Complex sentence: Eventually, however, a boy (whose wallet she had stolen) discovers her belongings in the library and Maggie runs away, eventually to find her long-lost grandmother, whose old letters she had discovered previously in her father's desk.
Split into simpler sentences: Eventually, however, a boy (whose wallet she had stolen) discovers her belongings in the library. She travels to find her long-lost grandmother, whose old letters she had discovered previously in her father's desk.