I think you are wrong
Hyunsung and Hyunmin are half brothers since Hyunmin's Mother is Myohee's mother too. She gave birth to myohee even before she married Hyunsung's Father...So technically Hyunmin and Myohee are the half siblings...
Tho I get your point since Myohee's mother and Hyunmin's father are married that made them "Half siblings" but since they are not blood related and met/dated before they knew who Myohee's mother was it is legal
The FL (Myohee) is not the MLs (Hyunsung) half sister. They are not related by blood. However, Hyungsung has a half-brother whose mother is also Myohee’s mother. So, both main character’s have a younger half brother, but aren’t biologically related.
A visual may help:
Hyungsung (ML) = his father = Hyunmin (half brother).
Myohee (FL) = her mother (MLs step-mother) = Hyunmin (half brother).
SPOILER!!
Honestly I’m still shocked how the female lead turned out to be the male lead’s half sister and the whole cast just accepted it. The story was good until chapter 65-70 but then it just started going downhill, with unnecessary plot and questions/important lore where just quickly explained and answered in the last special episodes. I give this a 5.5 or 6/10 (taking out the lowkey inc3st trope)