Ditto what seekrit said.
No multiples in my family whatsoever but still here I am and have had 3 multiple pregnancies. Personally I think it's just the luck of the draw.
Just a random fact but did you know that it is possible and very likely to have identical twins if you have IVF even if only one was transferred? The egg is more susceptible to splitting. Interesting!
ETA - Mine were naturally conceived (ie no medical intervention at all) just something I found out about not too long ago and thought I would share.