* * 1/2"BETWEEN THE LINES." 5 Star. RCA. Philadelphia-based Nick Martinelli produced the first hit records by this group of black British siblings, and he had a keen sense of how to show them off in spiffy fashion. Rather than continue that winning formula, the group teams with other producers--including the quintet's father, Buster Pearson--on its third album. While the result is professional and slick, it's not as fresh-sounding or as fun as the group's veddy veddy high-toned version of Philly Soul. "Strong as Steel" shimmers with a well-polished gleam and the softly-stated, low-key sexiness of "Knock Twice" is pleasing. Cuts like "Somewhere Somebody" and "Made Out of Love" don't rock out as strongly as they would in the hands of a more go-for-broke producer.