Complex sentence: But in December 1989 they got together and Sarah treat Robert like her own but Jack's 24-year-old son Jackie didn't like Sarah and tried to get Robert away from her any time he could.
Split into simpler sentences: But in December 1989 they got together and Sarah treated Robert like her own. However, Jack's 24-year-old son Jackie didn't like Sarah and tried to get Robert away from her any time he could.