fraternal twins are hereditary (means two eggs at ovulation). This only occurs on a maternal lineage.
identical twins are pure luck (one egg, one sperm, splits within two weeks of conception)

The older you get and the more pregnancies you have the higher your chances of twinning.

Apparently their is a greater chance (albeit slight) if you are overweight.

Breastfeeding when you fall pregnant may also increse the likelyhood.

Lots of factors involved, no simple answer.

For you,, the big factor is that twins appear on your maternal line, and that you have already concieved twins. You may have the 'hyperovulation' gene. This would mean fraternal twins. There is only one way to find out if you are going to have twins again!

Good Luck