I wouldn't recommend deliberately exposing your child to chickenpox as other posters have mentioned, there can be complications, although these are relatively rare. As the incidence of CP in the UK is high, it's quite likely they will become naturally infected at some point in childhood.
There has been some evidence to suggest that early CP infection is linked to an increased incidence of shingles at an earlier age. On the flip side to this, primary infection in adults is associated with severe disease, mainly pneumonia.
Some of the comments on here are incorrect. Slapped cheek (parvovirus) does not give cross immunity to chickenpox. You can have an asymptomatic infection, so may have acquired immunity this way without realising you ever had CP.
You can't get a more 'severe' infection from increased exposure from a sibling. Repeated exposure is actually a good thing as you keep boosting the immune response. There are individual factors which can make an infection more/less severe.
The vaccine has been used in the USA for over 10 years and has proven to be safe and effective. The only reason we do not give it universally in the UK is because of the cost/benefit ratio. In short, my advice would be to vaccinate. I know this may be controversial to say (and I'm completely pro choice) but vaccinations full of 'evil toxins' have been one of the biggest contributions to public health after clean water and sanitation! We can thank these (horrible) vaccines for eliminating small pox, nearly eliminating polio and reducing the incidence of many other diseases.
I have 3 DC and I would have vaccinated them if I knew then what I know now.