Thank you to both for the confirmation. As the game moved forward, it became clear to me that they had the same father but different mothers.
However, bearing that in mind there were a couple of confusing or weird moments in the story, in my opinion.
For example, shortly after reconnecting the MC thinks "When I first met her she was so little, and now look at her!".
When I first
met her?

If she's his younger half-sister, that's a weird way to put it because obviously he has known her since she was born. The way that sentence puts it, it sounds as if they were step-siblings who first met when their parents got together.
But this is the most troublesome part of the whole half-sister topic:
If they have the same father and their parents were married, how in the world can they have different last names?


I would get it if the father and the mother never got married, he then disappeared during pregnancy, so when she was born her mother became a single mom and decided to give her her own maiden's name. The mom could have taken care of the MC too, who kept his father's last name while Ashley had her mom's name.
However, it's been said that there was a wedding, and the father was around for a while when Ash was a little kid. She must have taken her father's last name just like her brother. What am I missing here?