About Shinmai No Testment Ova

Fieryx said:
If you haven't read the LN, I'd advised you to not read any further if you don't want to be spoiled.

[spoiler] So you've probably have seen the OVA and have watched the "hot" scene between Chisato and Toujou. But then I find out that Chisato is Toujou's Aunt. But then I found that their not blood-related and that Toujou's mother and Chisato were best friends which is why they call each-other sisters. So would Chisato be Toujou's adopted aunt basically? I think it's a bit weird since Chisato is deeply in love with his so-called nephew.