This just happened to me. I had a miscarriage in October that was devastating (I had wanted this child for two years, wasn't sure we'd ever have it, and am pretty sure that we won't try again; anyway . . . ). My brother and his wife called me about 2 days before we were to go to their house for Christmas to tell me they're expecting (the salt in the wound was that they had not planned to have anymore, this is an "oops" and they're not even sure they're happy about it; oh well . . . ). So, having been through this, here's my advice.
First, Don't put off telling them. Much worse than being told, would be finding out that people in my life thought they had to hide something from me; even worse if other people were being told behind my back. Second, try to tell them at a time and place where they can go have time to react and process the info in private. If they are like my family, they are going to be truly happy for you, but they also may be very angry at God, the world, everything about their situation. I was SOOO glad that my brother had the wisdom to call me ahead of our trip. AFter 24-36 hours of emotional tantrums in the privacy of my own home, I was fine to face the family and have a nice visit. If they had waited until I got to their house for a week, it would have been a nightmare. If you live nearby, and the phone feels inappropriate, maybe you could tell them when the whole extended family is not there or mention it at the end of a visit when they can gracefully leave without looking bad or offending people. Finally, when my brother called, he said nothing about worrying about how I would feel. I gathered later that my feelings had been a topic of concern and conversation among the family, but I didn't have to deal with that at the time.
Hope this helps. Congratulations on your coming baby!
V.