Complex sentence: It is their sea-faring twin brothers, Alf Laurel and Bert Hardy, who are dim-witted incompetents sailors aboard the "S.S Periwinkle".
Split into simpler sentences: It is their sea-faring twin brothers, Alf Laurel and Bert Hardy, who are dim-witted incompetents. Alf and Bert are sailors aboard the "S.S Periwinkle".