AFAIK with the patch she is definitely his half-sister. That's why I was confused when he said she isn't his step-sister.
IMHO they are both, marriage or not.
I thought with that patch installed their parents married and it was mentioned somewhere but I probably misremembered that.
I find it weird that he doesn't see her as his sister, even though he gets nightmares all the time and questions himself over their relationship every 5 minutes....
MC in this game is just weird.... what a little bitch...