No, they're not siblings. Their parents got married, they didn't grow up as siblings, and they only lived together for about a year in the same house. Most of the time, their parents weren't present, they didn't have a family dynamic, and to be honest, Geom's father never paid him any attention or had a present father role. They met at school when they were kids; they didn't grow up as stepsiblings or siblings. Som liked Geom since he was little; he didn't start liking him after their parents got married. They do have a half-brother in common, but they're both adults and don't live with their parents. The only reason they're not together is because of what their parents might say. But in this chapter, you realize their feelings for each other. So, they have to live miserable lives apart when they want to be together just to make their parents happy? Parents who started new families and moved on with their lives? So, they have to be unhappy for the parents to be happy? These parents, with whom Geom has no relationship and they don't even try to have a relationship with him.
But like the remarriage would make them siblings right? I guess an adoption may not have been carried forward but it's a hard double break because first things first the obvious is that they're gay. Then the second hard break is they are siblings by marriage. I get the fact Geom created the distance and all but as a third party looking in, your mind would immediately jump to "Aye you guys are siblings" even if it's not by blood. I guess the ends well would be both breaking away from the family unit - like disowning the parents. I dunno my brains failing to wrap around
They would only be siblings if they were raised as siblings and if they themselves wanted that bond. They are a blended family; in this case, they both share a brother (or half-brother). Som recognizes him as a brother and has a brotherly relationship with him because he has always been present. Geom doesn't know him and has no relationship with him at the moment.
Som sees Geom's father as Geom's father and his mother's husband, not as his own father. Geom doesn't have a paternal relationship with his own father, since he didn't pay much attention to him as a child, and even now as an adult, he doesn't have much of a relationship with him either. He also has no relationship with Som's mother or doesn't see her as his own mother.
Obviously, in this story, the two problems are that they're gay and that they're a blended family with a brother in common. This raises the question of whether they're family, and that's why they're not together. However, the fact that they love each other and want to be together creates a "stronger" bond than their parents being married.
At least, that's how I see it because in my own case, I have siblings, and my parents are divorced. They both remarried people who already have children of their own, like Som and Geom, and I met those children as an adult (I didn't grow up with them). We're a blended family; we spend birthdays, holidays, and lunches together, and we have a good relationship, but I don't consider them my siblings. Perhaps it's because we're all adults now, and growing up together as a family didn't create that bond, but we are a close-knit family.
Fact. Also I'm just of the mind that if you present publicly that you're remarried and have so many children, it's a sealed deal. You can't day you remarried, but our children are individuals aside and can do what they want. The new sibling complicates it further like.... those are his brothers. Ugh lol I won't think too far. It's just a trope I don't like

I will see it to the end but they are brothers. Why. No blood I get it but how do you navigate this? I'm like confused. It's a trope I reaaaally don't like but the art and story telling are good