I know that but I still think that this sentence has been added for a reason. Just like the whole conversation about, if Monica had ever seen MC's bio mom while pregnant.
In the end, there are a lot of reasons for what could have happened. For one, CheekyGimp stated on page 4, that there are no CURRENT plans forDIRECT incest. That statement in itself leaves all options open. We are now 1255 pages further, the CURRENT situation might have changed. I mean, Monica really seems to have a problem with the thought, that MC and Katie have sex. If you go with Monica to the carnival while on the Katie route, she will tell you not to have sex with Katie at her house. In the night after the movie event, Monica checks on them and made sure that there is no sex. If you do the same with Jenna, then the only thing that Monica will say is, that Jenna shouldn't get pregnant while in college.
Then this whole conversation about Jenna's body type being like the one from Monica while Katie's body type is not. I am pretty sure that there is something about Katie that we don't know yet, not even Katie knows it herself, only Monica.
I mean, there could be other explanations as well, explanations that don't include incest. Maybe MC's dad was the way he was to MC because MC has not been his bio child.