What is the right age to tell our daughter about the son we put up for adoption?

I got pregnant at 17 and I relinquished him for adoption at birth. He is 10 years old this year, his (bio) father is now my husband, and we have a daughter who is a year and a half. We have a semi-open adoption in that I receive updates and photos every year, and he knows that he was adopted and knows about my husband and I. Haven't seen him since I gave birth to him though, and he and his family live on the other side of the country now. Obviously my daughter is way too young to understand but I'm just thinking about the future and wondering how and when I should explain it to her. Am I over thinking it? via /r/Parenting http://bit.ly/2Drs4XQ