To put your mind at rest a little maybe... I had my first scan when docs thought I was about 12-14 weeks because of the size of me.
Actually turned out that I was 8 weeks and both little twinnies were seen clear as anything and heard too and that was 8 years ago!
It was also spotted that I was having twins at 6 weeks, when I had a scan due to complications before I lost them last Feb.
Modern technology has advanced quite a lot now!
As for your OH having twins in the family, he doesn't affect it I'm afraid.
Identical twins are a random chance of an egg splitting into two and Non Identical twins comes from you actually dropping more than one egg.
This can be hereditry in women and is in my family but not in men.
Only the woman carries the hereditry twin gene.
Looking at twin chances, I was working it out and thinking about being scared of having twins... apparently I have 5 times more chance of having twins than one... that is scary and I'm still trying
Good luck to you no matter how many you end up with and if it does turn out to be two believe me you get double the love back for all of your hard work