You are playing as Bethany (which is a direct hint on the Daz3D model, by the way), the mother of Charlie and Eva. Charlie and Eva were pretty close, too close for Beth and Alex, who Beth is married to. So they send Charlie to California, to live with Beth‘s brother Leo. After one year he returns, and the story starts there.
To say it with Beth‘s words: „I thought, if he lives with my brother for a few months, everything will be alright and their feelings would settle. Yeah right… Let’s just say they did in a way.“