While this is true I am still thinking that Sarah and Mike while having the same father may have different mothers cause unlike Sarah Mike holds no resemblence to Lucy.
Not to mention when we saw Lucy in front of Mike's home we noticed that she didn't get overly emotional until he mentioned Jack and Sarah living there with the two of them. Where as when it came to Mike living with Maria Lucy was nowhere near as upset over this.
This piece also leads me to believe that Sarah is her child and not Mike.
But on another note when I read the dairy of Sarah during her chain I found it extremely weird that Sarah calls Mike her "Onii-Chan" When she is supposed to be the older sibling. It should be Otouto seeing as he is the younger sibling not Onii-Chan.
mmm well everything can be possible. Another possibility is that MC look like is father and not is mother. Ofc if Lucy is his mother. Another possibility is that MC look like is grandfather, or perhaps some other relative of is mother. Not always resembles their parents, but also to some ancestors. Ofc you can be right and she is only the mother of Sarah.
For me i really hope that she is their mother, because i like incest. And think of it make me really horny XD
But beggar can also introduce a new woman that is effective the mother of mike only. Surelly beggar do some plot where MC fuck is blood-related aunt. For now he fuck only is sister. Now she can be his full blood sister or half sister.
For the oni-chan think, well if you know the Japanese story they prefer to use oni-chan for the cute purpose.
I see many hentai comics where old or younger sister call the younger or older brother onichan. In few versions they use some different suffixes. An example of different suffix is: when the story is made in the past, like the sengoku era, where they brought a different respect to the older one. But in this case, I think it's more for the translation level that for other thing.
Returning to the theme, some new walkthrough? XD