There’s no way he ate her out that had to be because the angle

I think they were just pretending (I hope so)

Btw Dong is her half-brother (spoiler from novel). Although he didn't really go down on her but uhmm still...

So what was he really doing down there? What kind of relationship they have?

I dont think they have *that* sort of relationship but i think FL was anticipating MLs arrival so she got him to pretend so ML sees it and then goes back to his kingdom pretty much

Where do you read the novel?! Link?