Complex sentence: Jennifer rebels by becoming engaged to Frankie; however, she breaks the engagement when she learns of her mother and grandmother's mental problems, thinking she'd develop the same problems and pass them on to her own children.
Split into simpler sentences: Jennifer rebels by becoming engaged to Frankie, however she breaks the engagement when she learns of her mother and grandmothers mental problems. Thinking she'd develop the same problems and pass it on to her own children.