guys is this incest?

No I don't think they are I believe they both have different mother's cause the mc did say,"his mother" meaning they probably don't have the same mother

Yeah they don't have the same mother. But do they have the same father?

I mean I guess so unless he was adopted, but they'll just be step siblings which isn't incest cause their not biological to that one parent

If they have the same father that means they share the same blood so it will definitely be incest. But someone told me that man isn't the uke's father. That man actualky killed the uke's father and raped and married his mother. That is why the man beats him so much. So it is not incest.