Hi there, I know exactly where you are coming from. My sister-n-law was trying for a long time to get pregnant, invitro and all. With no success.
When we found out we were pregnant I wanted to wait to tell her, but my husband was like, "this is my sister, I want to tell her" well, she cried on the phone when he told her. I felt so bad, but I also felt hurt because of her reaction.
Either way you can't win. Telling her or not. If you don't tell her she is going to think you were hiding it from her. I ended up losing that baby at 13 weeks, but I got pregnant again, but this time we waited 12 weeks before we said anything to anyone, but she knew we were trying so it was not a complete surprise, so she took it ok, at least in front of us. And her and her husband decided to not have kids. Now she is an Aunt and loves it.
But in your case (and it depends on the type of relationship you have with her) I suggest you just sit down with her and tell her, one on one. Honesty is the best policy. Tell her how concerned you were about telling her, and encourage her to keep trying and that it will work for her one day when it is meant to be. She may get upset, and that is only natural and expected, but if she really cares about you she will also be happy for you.
Well, I hope this helped you some.
Good luck and congratulations!
Don't let this bringyour spirits down, you have another little one on the way!