To define the word sibling we must first look to the Greek words aderfos ((αδερφός) and aderfi (αδερφή). Siblings are defined as those who come from the same parents or have been adopted into the family. A sibling may be a brother or a sister or even both. Some siblings are protective while others may need protection. Such is the case of Kyou Fujibayashi in Clannad. Kyou has a younger twin sister named Ryou and is very protective of her, going so far as to throw a dictionary or any other object at a person she believes to be bullying Ryou.
Kagome Higuarashi is similar with her little brother Sota. In episode 11 Terror of the Ancient Noh Mask, Kagome places herself in danger to rescue Sota from the Noh mask, urging him to run to the well and get Inuyasha. Despite arguing constantly throughout the series, sibling protection does take place. Even Sesshomaru protected Inuyasha from getting killed in Swords of an Honorable Ruler, despite his arrogant answer that only he reserved the right to kill Inuyasha.
Birth order does not seem to matter to which of the sibling should be protective and which should not be. Alphonse Alric, despite being younger, has shown his protectiveness of his older brother Ed, even going so far as to put Ed in his place if he thinks Ed has foolishly risked his life. Likewise Ed is also protective of Alphonse, even going as far as to sacrificing an arm and a leg for the cost of his brother's soul.
Anime has hit the head on the nail through stories that introduce sibling behavior. We can see from watching shows such as Rosario Vampire, Inuyasha, Full Metal Alchemist, Clannad, Code Geass, Akuma de Sourou, My Neighbor Totoro, and Grave of the Fireflies how siblings behave very similarly to that of siblings in real life. This is because manga writers have been able to see just how sibling relationship is maintained throughout a person's life by looking through the glass. And thanks to anime we've never felt so understanding of our siblings.