[header] How to deal with adult sibling bullying [title] Look for signs of jealousy or resentment. [step] Often, the underlying cause of bullying is feelings of inadequacy when compared to the other sibling. Perhaps your brother or sister thought your parents liked you more, and they became jealous as a result. 
Or your sibling may feel that you were more popular growing up or that you had an easier life. Think about what your sibling has experienced in their life and if that might contribute to them being angry or resentful.