They're not related by blood. The uke's mom is simply friends with the seme's parents and often visits the castle (she might be somehow connected to the Royal family like a distant cousin of the king or something but that's about it)
Omg tysm for clarifying I was so confused at first when I read the chapter
I think it's their culture. Girls call even their crush "Oppa" which technically means elder brother but not really. Same goes for "Hyung" in gay couple (as I've noticed)
In chapter 21 it says that mc’s mom left mc and his dad and went with the ml. DOES THAT MEWN THEY SHARE THE SAME MOM??!?BOTH MC AND ML????