Older siblings can have a huge influence on middle school students. A middle school student with an older brother or sister of high school age will be very much influenced by that sibling. But the way in which they are influenced is directly related to whether or not the genders match. If a middle school boy has an older brother, he will model his behavior to mimic his brother. If the older brother is good academically, then the younger brother will also strive for good grades. If the older brother does well in sports or the arts, the younger brother will also try to do well in those areas. On the other hand, if the older brother does very poorly academically or is not at all interested in school, the younger brother will often have a similar attitude.
However, if the older sibling of the middle school boy is a sister we see the exact opposite. In this instance the middle school boy will strive to be different from his older sister. If she is a serious student with good grades, you might find that he adopts an attitude of school disinterest. If she excels in sports, he may decide to take no interest in sports whatsoever and pursue something different.
This scenario plays out regardless of the gender of the middle school student. When the gender of the older sibling matches, the child tends to pattern their own behavior, interests, and goals after the older sibling. When the gender of the older sibling does not match, then an attempt is made to follow a different pathway than that sibling.