Complex sentence: Martin I of Sicily (c.1374--25 July 1409), "The Younger", King of Sicily (1390-1409), was the grandson of Peter IV of Aragon and Leonora of Sicily, and nephew of John I of Aragon.
Split into simpler sentences: Martin I of Sicily (c.1374--25 July 1409), called "The Younger", was King of Sicily from 1390 to 1409. Martin was the grandson of Peter IV of Aragon and Leonora of Sicily, and nephew of John I of Aragon.