Yup just want to second what Magpie said - it comes down the mothers line - my hubby has twins in his family so i was worried last time and researched it lol. Luckily there are no twins on my side of the family at all! Although - also like Magpie said, everyone has the chance of having identical twins. Eeeeeeek!
Just to agree with the other ladies, ID twins are just nature so doesn't matter if in the family or not but there is thinking that non ID twins on the Mum's side may be hereditary x