I'm not sure about the question you are asking. What type of Visa is he applying for? Is he sponsoring his brothers and sisters? If so, then he does need to get that information from them if the brothers & sisters would like to be sponsored.
Or is this from the family information section you need to fill out when you lodge the application? I also have some relatives that I've lost contact with and just simply wrote down their names. I remember there's also a box where you can add your own notes and I wrote that I have lost contact with them and will not be sponsoring them as secondary applicants on my visa application. Is that what you are asking about?