My Mum had two sets of twins, after the first set you are more likely to have another set apparently. Older women have more twins too. Statistically though sorry I can't help you.
There also seems to be hot-spots. We live in Hereford and their are twins everywhere. The local primary school (180 children maximum) has at least four sets at the moment but there have been years when there have been more. (One of those sets are my very own thing 1 and thing 2.)
Also female fraternal twins are more likely to have twins and you are more likely to have twins the more children you have (which I found out AFTER I fell pregnant for the fourth time).