In one scene they look exactly alike like I mean escaped the womb on the same day alike from the same mother.
usually in manhwa though, they use different hair or eye color to differentiate characters. Eg. having the same shade of red hair or eyes could be a hint that two characters are related. In the same way, an artist can draw two characters with two different shades of blue hair to show they're not related at all. In this manhwa, they didn't do that with Eric and Marianne (forgot how to spell her name). Not to mention that the faces the artists draw for different characters tends to look similar
they don’t seem like siblings at all it’s just the hair color .... me and my boyfriend have black hair this doesn’t make us look like siblings....