So sweet that you thought of her in that way. Really, it is.
I shared my first pregnancy with two friends. It was great to have someone to talk about everything with.
With my second pregnancy, I wanted it to MYSELF! Lol. I know it may sound silly. In truth, the only person I would have wanted to be pregnant with was my sister. It wasn't her time, but I would have LOVED to share it with her. And to have the cousins be that close in age - perfect.
I agree with the other poster. I would tell her before you tell anyone else. I would even tell her that you feel a little guilty, I'm sure she will be happy to share it with you, though.
Congrats to you both!!