Best way to find long lost family?
The good news is, his sisters are not likely to have the same reaction as the brother. If your dad can find just one sister, she will probably connect him to the others. Maybe you could hire a private detective. Give him the info on the brother that has already been found, and the heads up that that brother doesn't want to be contacted, and let the PI find out where that brother's sisters are.
Of course it isnt an invasion of privacy, its his family. Maybe one of his sisters still isn't married and can tell him where his other sisters are.