Okay so why did no one tell me they are basically siblings

In this case ML and MC were not that close until they study in the same classroom (because thought they hated each other), didn't interact to appear brothers behavior (even though both lived in the same house), due to the misunderstanding and because they actually aren't (biologically nor legally).

Even when they reconcile, neither Taesung nor Haebom considered each other brothers, but friends. Only Taesung's mother considers them both as her's children but she never forced them to call or consider themselves brothers, just family.