what’s with all the incest in these stories, can they have a normal sibling relationship...

I thought so too, but I believe the author meant it as a way to say that Derrick never saw Penelope as a sister—he felt no issue liking her because she was never his sister. Obviously it was still wrong since, to the outside world at least, Penelope was Yvonne; Penelope was a “some sort” of sister, so he hid it out of shame. Compared to his actual sister, Yvonne, who he just cares about out of sibling love, blood, and guilt.