One night won't hurt, but it might make you feel a bit sick being cooped up in a room with smokers I smoked through 2 pregnancies and had very healthy, fat little babies... not that that's any justification, but if it reassures you that you won't do any lasting damage then I hope it helps.
Are your parents accommodating enough to smoke outdoors for the duration of your stay? I'm sure a kind word to them about it before you arrive would go a long way, I booted DH out of the house to smoke the minute we found out we were expecting #1 and he didn't mind at all.

As a PP said, you cop worse toxins from walking down a busy street in broad daylight (mainly traffic exhaust etc), so don't stress