Yeah, it just ends with the Mom NTR'ing the sister. In all honesty, I like the sister better. It just seems that what made this story popular was abandoned for a rather generic mother-son narrative. And the detail in this chapter pales in comparison to the first one. Though, that's just an opinion.
In part one, it's hinted that Dylan is attracted to his mother, and Katie is attracted to her father, but not as much as they are attracted to each other. At worst, I imagined it was going to be some family orgy with swapping if the father was included. I agree with OldGreyGeek, the story would have done better to include both Katie and their mom.
I don't think there will be a part 3. Unlike Chapter 1 which ended with 'To be continued...', this chapter ends with 'The End.' It follows Mr.Foxx's M.O. He doesn't really do extended works, and when he does it's just one or two more chapters. The only work of his that I believe has more than two chapters is 'Mom's Affair.' With that said, it doesn't surprise me if 'Discovery' indeed ends here.