Hustler to Rob Kardashian: Really Wanna Screw Your Family?

Rob Kardashian and Blac Chyna are getting the chance to star in a brand new Kardashian sex tape ... with an earning potential of 7 figures.

TMZ obtained the letter sent from Hustler honcho, Larry Flynt, to Rob and Blac -- asking them to consider shooting a video for the company's celebrity sex tape division.

Flynt says Hustler is looking to be a major player in celebrity porn ... and thinks R&B's fan base will benefit all involved.

He promises they could be looking at earnings of a million dollars or more.