Suk-hyun isn't jealous of In-bum. It's more like he's jealous of Inbum's relationship with Jong-nam. In-bum has potential to be Suk-hyun's rival for Jong-nam's love. Suk-hyun fears that In-bum might pursue a relationship with Jong-nam. And since Jong-nam doesn't want to hurt the families, Suk-hyun fears that she might just go with In-bum.
I agree, evelynk. He may be a little on the skinny side, but Mr. Choi is rich, polite, handsome, educated, great skin, dimples and a smile to die for. No worries about his future. Definitely the match a mother would want. Sukhun may be rich, too, but I'd take Mr. Choi any day.