Sibling analysis is sued to determine if two individuals are brothers, sisters, or brother and sister.  Two individuals born of the same biological parents share much more of their DNA than individuals that are not related to one another.  On average full siblings (share both biological parents) have 50% of their DNA in common, half siblings (share only one biological parent) have 25% of their DNA in common.  A statistical determination can be made as to whether two people are siblings based on the amount and specific type of DNA they have in common.