I've always heard that twins skip a generation, but are usually carried through maternally, so the daughter or grand daughter would be more likely to have a twin rather than a son or grandson. I may be completly making that up though. Twins run on both sides of my wifes family, but thankfully we only have two boys (5 & 1 1/2)...They do look a lot alike though

About morning sickness, my wife never had it either. As a matter of fact she was 5 1/2 month before we found out....it's a long story.
Crazy thing is, a month before we found out we went on a trip to Ireland with some friends and we were boozin' it up on Guinness and smokes the whole time. We were hardly sober the whole trip. And since we were newly weds, we were going out every weekend as well. Thankfully my son is completely healthy and is pretty advanced for his age.
We joke about all the Guinness adds that say "Guinness for Strength" or "Guinness for Good Health". Ours is "Guiness for a healthy and intelligent baby".
So my fatherly advice...buy your wife a six pack of Guiness and some smokes...(Kidding of course)
Actually, sleep as much as you can now, and remember once you have a kid, life is no longer about what you need, but what you need to give.
It's all good and congrats. The best is when you can identify those little bits of you in your own child, the good and the bad.