I'm just a lurker, for now, but my DH and I went through a similar situation when we found out we were expecting our DS last year.
DH's sister had just found out that there was nothing more they could do with regards to fertility treatments (not sure of the details, but it took her a while to conceive their son, with the help of fertility treatments, and he is going to be 6 this year). So, basically, a few weeks before we found out, they had come to the realization that they probably wouldn't have any more children, and she was pretty devastated, to say the least. They were recommending a hysterectomy when she was done having children, so double whammy.
DH and I talked about how we were going to break the news, and we decided on this: DH went over to his sister's house (without me), and just sat them down and let them know. He basically said, "I know that you're happy for us, but I wanted to let you know privately so that you could react however you wanted without an audience. We know that the news you got might make you react in a way that you would not want people to see. Not exactly those words, but you get the concept. She of course cried, but she was very grateful that we handled it that way. She said, obviously, that she was very happy for us, but it was still just a little raw since she was still in the grieving process.
Hope that helps in your decision. It's a tough one, but just remember that no matter how the immediate reaction to the news goes, that they most likely are very happy for you both.