Yoon Kyung-ho is the aunt, Choi Dae-hoon is the dad, and So Ji-sub is the grandfather (Chief Kim making-of video)

Seo Su-min revealed that Yoon Kyung-ho is like an older aunt and Choi Dae-hoon is like a father.
When Yoon Kyung-ho asked about So Ji-sub, "What about you, hyung?", Seo Su-min cautiously replied, "In terms of personality... a grandfather," making the three of them burst into laughter. So Ji-sub laughed, saying, "Not even a dad, but a grandfather," and Seo Su-min clarified, "No, his personality! He's sweet. My grandfather is sweet." It looks so nice to see! Hahaha.