royalweddingpt2042911_01_X17230WM.jpgKate Middleton's gorgeous sister Pippa has been given the offer of a lifetime from porn company Vivid Entertainment ... $5 Million to appear in just ONE explicit sex scene.

Vivid's CEO Steven Hirsch sent Pippa Middleton a personal letter with an offer to star in one of his upcoming films. In the note, Steven writes:

    "As far as I was concerned, you were the star of the recent Royal Wedding. As I watched a broadcast of the event I couldn't help but think that with your beauty and attitude, you could be an enormously successful adult star. For just one explicit scene I would be pleased to offer you $5-million and, or course, you would have your choice of partner. "

The letter goes on to say Kate's brother James can make a milllion, too. Hirsch adds:

    "If you would like to bring your brother James along, he could appear in a separate scene for another $1-million."

Vivid's rep tells X17online, "We haven't heard back yet."