Here's a trailer for a game that someone in the comments of this article first brought up. I thought about the best way to phrase the title of this post, but in the end, the name of the game itself says it all.
Listen to Me, Girls. I Am Your Father! is a light novel and manga series that Namco Bandai has seen fit to adapt to the PSP as a visual novel. Despite the title, you aren't actually these little girls' father. They are your sister's kids, and in fact, only one is her biological daughter. Whatever, though. You can cut it however finely you want; it's obvious that the humor is derived from the incestuous and borderline pedophilic undertones.
The special edition even comes with a bonus "Sleep with Dad" pack. Make of that what you will. Does it bother me? Nope! I can only laugh. Cultural differences: what a riot!