I agree with Ellie, I think it would be best to make sure you get her on her own to tell her. My worst one finding out was earlier this year when we were about to do another IVF cycle and my brother was having a bd party. All my SIL family already knew they were pg and were saying little things so I had my suspicions then my mum just said to me in front of everyone!!!!! I was quite miffed and more than a little upset. My brother later on got me on our own to tell me but it was too late (he didnt know that).
My advice it let her know before others start saying things caust this news travels FAST! she may be upset for herself but will be happy for you and really grateful you have been so sensitive to her.
GL