i think twins are like 1 in 500000 chance, a family history of FRATERNAL twins puts you at a higher risk (along with a few other things like age weight etc) but it doesnt mean you WILL, also if your talking about a family history of identical twins, thats just a fluke, it may SEEM to run in a family but gentics dont play any role in that
that being said, i doubt the ultrasound tech missed twins....that is REALLY hard to miss