Complex sentence: He overhears Cristina and Eliud arguing, and concludes that Cristina is jealous of the friendship between Eliud and Elis, which makes Elis neglect Cristina, his betrothed from childhood.
Split into simpler sentences: He overhears Cristina and Eliud arguing, and mistakes their meaning. He concludes that Cristina is jealous of the friendship between Eliud and Elis, which makes Elis neglect Cristina, his betrothed from childhood.