Someone remind me. Those two are like half siblings, right?

Not half siblings. They are nothing to each other in that sort of relation. When Eunhyuk‘s dad was married to Eunhyuk‘s mom he started an affair with Raim‘s mom who had given birth to Raim since before the affair started they are almost the same age I think Raim is 1 year younger. Other than that I think Raim‘s mom is either dead or left her so that’s why she got adopted by Sooae‘s parents