Congratulations, and I hope everything goes smoothly.
As for that 1 in 3 statistic, that's what comes out if you include every case where a sperm and an egg fuse as a pregnancy--even the ones that don't implant in the uterine lining, and other situations where the woman never knows or suspects a pregnancy, along with miscarriages in the first couple of weeks, which are only known about if the woman did a very early pregnancy test. (I think it involves statistics and stuff about very early development, since obviously you can't survey women for "have you ever been pregnant and miscarried without knowing it?" Well, not and get anything resembling a useful answer.)
I would add that you should feel no obligation whatsoever to give those relatives any further information about your pregnancy, such as gender (if you choose to find out about that), the names you're considering, or anything about your own health. You aren't obliged not to tell them, either, but it sounds as though it might be less stressful to answer all questions with "I don't want to tell you anything that might disappoint you later."