Blood is thicker than water, however do not underestimate the ability of the human heart to build connections that are every bit as strong as blood.
What I'm saying is that adoption can be an even stronger bond because it is a relationship that is chosen. "You wanted me," is what the heart understeands. Very powerful.
By way of example, look at the powerful connections people have to country. Natives in a country are loyal by birth, but often an immigrant can appreciate a country even more because they appreciate a country so much.