I think all three of them are 1/2 siblings, but MC is the only one with a “commoner” mother/not an official wife so he was treated as an illegitimate child.
But MC does call her mother aunt, she was the only one who treated him normally like her own child, so I could be she’s a cousin but since they all grew up together they’re “siblings”.
The other guy and MC are definitely brothers by blood though.
Thanks for explaining. (◍•ᴗ•◍)
In the Korean language children born to the same generation within the same family are referred to with a word that means sibling, not cousin. That's because a married couple isn't considered to be a separate family unit after the wedding like in Western culture. Instead the person who marries in (usually the wife) is moved to the family register of their in-laws. However, their last name remains the same because changing yours to your spouse's is a Western culture thing. Sometimes extended families even live together within the same central family house.
In this particular instance, Si-Moon and Si-Hyeok are half siblings who have the same father and Yujeong is their cousin. All of them are in the same generation of their family, and from what I've read I think they were also raised together in the main family house. I'm not 100% certain whether Lee or Kim is the main family name, but either way one of their dads married into his wife's family and his kid(s) carry his last name instead.
The chapters are kinda confusing. Is it because of the translation? And the Blondie is his sister but her surname is different, is she his half sister? But then how is he an illegitimate child? It's said that his mother was the "other woman" then his father's surname should be Lee but he is Kim. But his younger brother is also Kim so is his brother also illegitimate child? The family tree is confusing me. I think she's his cousin not half sibling. Or not! (