How can we find an adopted sibling urgently?

I think that the best thing to do is to get in touch with one of the agencies shown on this link. a href="http://www.billsimpson.com/Srch.html" rel="nofollow"http://www.billsimpson.com/Srch.html/a It is not going to be easy, but perhaps they can make some helpful suggestions. Since you have no idea of the name used by your brother-in-law, I don't think that the other suggestions are going to help.