The two most important strengths in inquiry are organization and curiosity. They seem like obvious and unimportant factors but, once you break them down, you realize their true value.
Organization is a problem solving skill that helps you to categorize, classify, and give hierarchical placement to information. Since social science is a field entirely based on first and secondary data the categorization and organization of information is essential to determine the direction that the interview will take.
For example, you should divide the interview into sub-categories. Each of these categories should represent each sub-field of social science. You should select which of these sub-categories will make up the "theme" of your interview. In other words, knowing these categories gives you choices for your questions.
After you draw questions from sub-categories, you should employ your natural curiosity.
Natural curiosity exists in all individuals, but to make the best of it you must trigger it with good motivators. In your case, the best motivation is that you will be facing primary sources: your grandparents. A second set of motivators are the gains for your own sake that you can make: how will your questions open windows into your own views of yourself as a member of that family? How will you feel as the grand daughter of people who lived through the things that they lived through? How will that echo in your future? What things will you keep private, and what things will you publish in your final report?
With natural curiosity, organization, and a clear understanding of the purpose and goal of the interview, you would have no problems discerning what information will matter most for you.
Remember also that, since these are your family members, there is a risk of subjectivity that unfortunately may drive your interview astray. Review your questions and ensure that you are speaking objectively at all times, and that none of the questions are related to you. Assure the interviewees that your report is not for personal insight but for the purpose of conducting social science research in the sub-fields of _________ (blank). This objective approach will also help you absorb the information without bias and without personal judgement to what they are going to tell you.
Again, build your interview as if it was field research: IRB forms, consent forms, confidentiality forms, and always offer the questions in writing to the interviewees prior to the interview so that they can feel confident in what they want to respond.