Just to be clear on what a Sibling is: A sibling is one of two or more individuals having one or both parents in common. A female sibling is a sister and a male sibling is a brother. In most societies throughout the world, siblings often grow up together, thereby facilitating the development of strong emotional bonds <Wikipedia> Now that we are clear: The definition is provided to give you an idea of how important it is to interview your Sibling(s). Whether he/she be your whole sibling or your half. He/She may have a wealth of information to share with you, especially if they are much older than you. Do your project a favor and set each one of them up for an interview.
If you have multiple Siblings, interview them separately, then together as a group. Sometimes an older Sibling can help another Sibling to jog their memory on certain events.