Finances allowing, I strongly encourage you to adopt another African American child. As your younger daughter grows up, it will be invaluable for her to have a sibling to share things with who has more in common with her. Just as children share things with each other they will not share with parents, there will be things your younger daughter will not share with her older sister for at least three reasons (age, ethnicity and adoption status), even if they are very close.
Beyond being the only African American in your family, your child will most likely be the only African American in most social settings beyond your home. Making your home more of a "haven" for your younger daughter includes supplying her with an ally there as well. Your daughter will bring home issues of adoption (family tree day at school, not having infant photos) and racism (children and teachers being prejudiced, and later issues of dating and body image in a white culture). Having a sibling to share these things with who also experiences them is critical to development. This is not to say your older daughter can't serve some of these functions, but the reality is she can only empathize, as she will not have similar experiences to share.
I have an adopted African American brother Chris, who is 7 month younger than I. He told me things he will not tell my parents to their dying day, partly because we both attempted to shield them from the ugly racism we experienced on a regular basis. Often, if the issue either of us brought to the other was serious enough, we encouraged the other to seek parental advice or intervention (racist principals, being physically threatened by police or neighbors, the list goes on).
In retrospect, Chris was the best decision on my parent's part in terms of making my childhood and early adolescence bearable. He was my best friend until 2nd grade. He was also a positive daily reminder that I was African American, and not alone in my family. While my parents see me as a special gift to them, I see their adding Chris to the family as a special gift to me. One for which I am eternally grateful.