Complex sentence: On her and Steve's wedding day, they were shocked to see Becky present and worried that she would cause a scene but Becky unnerves them by standing up to straighten her dress but doesn't interrupt the ceremony.
Split into simpler sentences: On her and Steve's wedding day, they are shocked to see Becky there and worry that she will cause a scene. Knowing this, Becky unnerves them by standing up to straighten her dress but doesn't interrupt the ceremony.