Complex sentence: In the series' third season, Danny began dating Claire Ellison; they eventually married and gained custody of Allison, his 5 year old niece while Joanie, his sister, was in rehabilitation for drug addiction.
Split into simpler sentences: In the series' third season, Danny began dating Claire Ellison. They eventually married and gained custody of Allison, his 5 year-old niece, while Joanie, his sister, was in rehab for drug addiction.