Hi C.,
First let me say Congratulations!!! Adoption is such a an amazing journey. We adopted our daughter at birth. She's been such an incredible blessing and joy. She's almost 2 now.
I completely understand how excited you are for your Aunt and Uncle. Every one should be excited for them and support them and share in their joy. However, as a couple of the other ladies have said, be cautiously optimistic. An adoption isn't "sure" until birth mother AND birth father relinquish their rights. Before that, they can change their mind and it could be devastating to your Aunt and Uncle. (I had a friend who had even taken the baby home only to have the birth father step in and assert his rights when the baby was a month old.)
My suggestion would be to send them a card or maybe flowers if the call went well and the birth mother chooses them. At that point, it should be an official "match". That will let your Aunt and Uncle know that their family is supporting them and excited for them.
My suggestion would also be to hold off on showers unless your Aunt and Uncle agree to it before any planning is done. Your best bet would be to talk to them and find out what THEY want regarding the timing of any showers. A baby shower is every one's first response to such amazing news and it is a natural and wonderful response. But it can also hold painful memories if their adoption is "disrupted". We asked our friends to hold off until after the baby was born and we were home. Because we were in another state when the baby was born (and had to stay for 2 weeks for all of the paperwork to go through between the 2 states), the birth mother's rights were relinquished and they had every thing started with the birth father. Our daughter was 6 weeks old when we had a shower for her. It was actually pretty great to do it that way because every one got to meet her.
A baby book is a wonderful idea. I did buy one for us before our daughter was born and took it with us for her birth. I kept it tucked away and didn't start writing in it until our birth mother's rights were relinquished. I found ours on Amazon. It is "My Family, My Journey: A Baby Book for Adoptive Families" by Zoe Francesca and Susie Ghahremani. I checked Amazon this morning and there are more now. 2 years ago that was the only one. Even on Amazon. It is wonderful because it is specifically for adoptive families and their journey.
Again, congratulations to your family. I wish them the best of luck in their adoption journey.