Complex sentence: Her mother Bonnie Plunkett (Allison Janney) is also a recovering drug and alcohol addict, and her 17-year-old daughter Violet (Sadie Calvano), who was born when Christy was 16, has become pregnant by her boyfriend Luke.
Split into simpler sentences: Her mother Bonnie Plunkett (Allison Janney) is also a recovering drug and alcohol addict. Her daughter, 17-year-old Violet (Sadie Calvano), who was born when Christy was 16, and herself become a teen mother by her boyfriend Luke.