It is important not to be insulted by lack of logic or common sense and those who have any "gray matters" will agree that this movie just doesn't work.
The problems lay in the direction, cast selections and lack of depth in the character building. The word comedy was very hard thing to say when i expect to laugh when these words are used. Let's look at the problems in direction/script.
Brother and sister both in their mid 30's seem to be well adjusted. They meet a complete stranger at a park and Heather Graham character walks up to her and asks the most intimate questions that even half sane person would be running the other way or at least scream for a police officer. He then awkwardly walks over and makes some stupid statements and she falls for him. Then after ONE date were they all go out together he falls in love with her and decides to get married in Vegas in a week's time???? Hello does anyone feel stupid yet? He goes out with thousands of women and he meets this one person who says about 10 words that WE see on the screen and he wants to marry her. Not only was there no chemistry it just doesn't make sense. Sure it's a romantic comedy and I want to believe it could, but the direction made it completely flat.
Now Heather falls head over heels with her too and when Heather Graham and Bridget Moynahan (very shallow character) kiss or more to the point it was sloppiest kiss ever that chemistry MIGHT be there. I found it unromantic and unfunny and while many say Heather cannot act i think the reality is Heather was clearly the wrong person for this role.
This was Sue Kramer debut as a director and to me it was just too much for her to chew. It would take a lot of craft to make this movie work and IMHO it could be done with better writers and casting and direction.