well no. they're not. bc kusaka's dad had a relationship with mizutani's mom and then mizutani's mom died so no. they could've been half brothers but they're not.
I think mizutani's child was from another man since he work as a prostitute, maybe he got pregnant when he ran away from the seme's dad, since they're just college students at that time
wait don't tell me they are HALF BROTHERS!!!!