[blparent] Talking about adoption?

[Pipi]

Hey y'all,
This will be long because I ramble.
I'm full of questions lately.
Last week an incedent happened and now that I look back, I didn't handle it in the way that I should have.
A little background for those that don't know. Savannah is adopted. I went to the hospital and was in the surgery room when she was born. I was the first to hold her and brought her straight home from the hospital. I've always been her mom. The birth parents have always been aunt and uncle.
I have a huge family and while all over the age of 8 or so know that Savannah is adopted, not all of the kids know who her birth parents are. 
Last week, I had nieces and nephews over and Savannah here. The topic of family came up somehow and one of my nephews was being a typical 13 year old pedantic literal boy and said that Savannah isn't really family because she is adopted. 
I understood what he meant, but the comment hurt me. I tried to explain at that point that even though she is adopted, she is still family, but then the little kids started in with "Savannah is adopted" I didn't know she was adopted" and things like this.
I got overwhelmed pretty quickly because Savannah was sitting right there during all of this. I changed the subject.
Looking back, I shouldn't have done this.
I have never planned to hide Savannah's adoption from her. I have talked to an aunt about when she told her children they were adopted and she said around 5. I guess I just figured I still had some time.
Now after doing a little bit of research online, it seems that I should have been telling her from birth. I'm not really sure how I should have incorporated her being adopted into our everyday lives. 
People have told me that she is too young to understand any of it. And I agree that she won't get the entire meaning of adoption at 3, but I don't want to handle the next incedent the way I did last week.
Does anyone have any resources on how to tell your child that they are adopted? Any known good preschool aged books on adoption? I have found a few but they are about a family going to another country to adopt. This will work, but I'd like to find something closer to our situation if possible.
For those who have adopted on here, when and how did you tell your children that are adopted?
Thank you for any help you can give.
Pipi