HFMD has been running rampant here in Singapore for the last few months. It's finally tapering off, though. I don't know about out there, but here in all the daycares, schools and anyplace with children's "classes" (i.e, Gymboree, etc.) the kids have to have their temp taken by the establishment before they're allowed in - if the child is running a fever, they are not allowed in.
As far was worrying, there's not much you can do about it now that your dh and dd have already been around it. So try not to get too stressed about it, mama! (I know, easier said than done)
With that said....now that you *know* your niece has it, I would probably avoid her until she is well again.
I'm no expert on HFMD, but I think the only concern (that may affect your pg)is the fever, if it got out of hand. The sores are no picnic, but I don't *think* they will affect your pg or your babe (but obviously, I'd check with your care provider).
Sorry I couldn't be more help to you - - I guess all I can say is if you start to notice sores on yourself (or dd, dh), or a fever for any of you, call your provider to discuss it's possible effects for you/babe.
I know it's pretty contagious, but I've managed to avoid getting it myself, even with all of ds's activities, and playground trips.
: So there's hope that you've dodged the bullet also! I'll keep my fingers crossed for you....