Do you know the adopted daughter's present name, or maiden name?
If so, try the Salvation Army - they are usually the quickest at finding "lost" relatives.
Going through the adoption authorities might well take longer than the time your sister- in-law has left. Would you consider promising to see that her daughter gets the information regarding the genetic condition if time runs out for your sister-in-law?
An association for sufferers from that particular condition may exist. If it does, get in touch with them. They may well have members who have specifically mentioned that they do not know the names of their birth parents due to being adopted. The society cannot give you their details, but might undertake to forward your sister-in-law's to women born in the same month and year as her daughter.
A dream car for my beloved mom
A dream car for my beloved mom


