What did dohu mean by our houses great demon king? And also who are they?

I thiiink he has a sister, if i remember right.

I think that part was mistranslated, also the dude is the guy from the film dept. from his uni and the girl is a senior from his dept.

Basically, Han Seojin is just a friend, so there's no blood relation. But her personality is starting to become similar to one of the family members at home (our 'demon king').

this is meaning that word. its not misstranslated

The 'demon king' basically refers to one of the irritating/annoying members of Dohu's family. And Han Seo-jin is starting to resemble them