Complex sentence: Afterwards, Dong-choon's mother finds when Man-sik's uncle (Eok-jo) was with her when the tsunami hit, she was worried about Dong-choon & Eok-jo was worried about Man-sik.
Split into simpler sentences: Afterwards, Dong-choon's mother finds out that Man-sik's uncle (Eok-jo) was with her when the tsunami hit. She was worried about Dong-choon while Eok-jo was worried about Man-sik.