adoption and the older bio child

We have a bio (CC) DS who's 7 years old, and adopted (AA) DD who's 2 years. The other day we were riding in the car, and DS is asking DD a lot of questions "Do you want this dinosaur?" "yes" "Do you want a drink?" "yes" "Do you want to play outside?" "yes" "Do you want me to be nice to you?" "yes". You get the idea. Then comes "Do you want your real mom?" Whoa! I almost drove off the road! We had a short discussion about the term "birthmom", and how I'm her "real" mom, etc. But, this caught me off guard. We've not had many discussions about DD's adoption, or her birth family. I hate to admit, when I think about those things, it's generally in the context of DD, since she's the one who was adopted, and isn't old enough to really understand, or have questions. But, DS is affected by the addition of an AA child to our family, since we now stand out in a crowd, and I'm not sure, he's never confirmed it, but I think he may be taking some teasing over having a black sister. So, what are some of the things you do for, or talk about, with your bio children? I plan to offer to answer any questions he may have. Can anyone recommend any good books, especially with children of different races in the same family? Another thing. DD's got some birth siblings, whom birthmom is raising. I've not put a lot of thought into how I'll explain that to her when she's older, but I think I need to be prepared with something for DS. Do I tell him she's got biological brothers and sisters? Do I say that her birthmom has other children? I don't want to spring it on him later, but, I don't want him to be the one to spring it on DD, either. Sorry, this feels long and convoluted. I'm just a little off-kilter today.