He's doing the right thing and a loving thing by adopting the child, but his sister is responding from her own devastation and unhappiness at not having a child. Adoption is hard, and when it's through the foster care system, there is sometimes a lot of grief between birth parents and foster parents. Additionally, the children are frequently traumatized from early abuse.

Foreign adoptions, on the other hand, can run into tens of thousand of dollars. So although he's doing a good thing, she is responding out of hurt and anguish over her own situation.