If I were you, I would only buy a gift set of skincare products from Clarins (something <=$100) for your future sister-in-law. Don't buy DV to her because she will not count it as the present for their future wedding. Then later on when they get married, I guess she will expect you to give them something as the wedding gift (such as a big fat check) - what are you going to do then? Tell her the DV is the gift? It would be very embarrassing. No way you will get away with it. So, no DV, just small decent gift and that is it. You can save your $500 - $800 for their wedding.

Btw, are you sure she will be your sister-in-law?