It's personal choice. Some people tell family as soon as they get the positive test because they'd want the support if anything went wrong. As far as I understand chances are good if you get to 9 weeks with heartbeat!
I guess a question would be what if you got your NHS screening back as high probability of fetal abnormalities after your 12 week scan? If it would make no difference to you or you'd be glad for the support (and they would be supportive) then go ahead and tell them. If they're very judgemental/ religious and would be against your choice then you might make a different decision.
If you see them a lot, get on well and you're feeling really unwell then they'll figure out something is up so may as well tell them rather than avoiding them!