
Probably the brother and sister is dark skin cause both work in the field, so they get a nice work body too. So it's not that they are big and dark skin is the cause. They work in the sun with a very demanding work that make them have muscle. So are muscular with a tan body.
Remember they are small Baron and not high rank nobles, less money, more freedom, probably they work their ass in the field to help or because they like it
You are just seeing your own prejudice.

Honey, you’re making up whatever to accept that this isn’t what it is. Yes, they are a baron family with less money than high ranking noble—but who said they own fields? There has been no background into Hemelias family to justify such designs. So STFU.
As of right now, they are drawn like that to make the FL seem like this “soft delicate flower,” amongst her family since she is the “baby” of the family. Its poor character design 101. The author is using these characters to bring in “comedy,” into this sequence. They are designed this way to make the FL standout. It’s supposed to be a joke. I just don’t find it amusing. There are other ways to bring comedy into the sequence. This just feels cheap.
Here is why I think it’s icky….
Mom is white. Dad is white.
Siblings are darker skin and drawn to look manly and ugly.
Why?
The author could have kept the skin white to look like the mother and father why darken them? And it feels HELLA deliberate.
I am so tired of authors from the East and Yellow Sea doing this. When is it going to end? I’m so tired of it. Ignorance is not an excuse. This is honestly their inner prejudice manifesting subconsciously in my opinion.
I can legit only think of one or two artists from SK who have not done dark skins dirty. To me dark skin characters rep —Thai, Vietnamese, Filipino, Middle Eastern/west Asia, etc. We all know they all Asian countries have a colorism problem.
This definitely gave me the ick. I can’t read anymore and I was getting tired of the 1st and 2nd couple. I stayed for the short hair girls love story. Which honestly seemed the best written. But I wont miss this story