Family is an important part of Chinese Culture. Where you sit within the family determines what level of clout you have and say in things that occur. Because of its importance, there is a whole language created to emphasis this.
In the English language, we refer to our siblings as simple brothers and sisters. We might go a step further by saying they are our "older" or "younger" brother or sister. But there isnt much credence of differentiation made because of this designation other than age.
In the Chinese language, the older vs the younger version of a sibling have entirely different names. An older brother is "ge ge," a hard g sound to symbolize the strength of this person as compared to the speaker. A younger brother is "de de," a much softer sound. Likewise, an older sister is "jie jie," with a hard sound to the word. A younger sister is "mei mei," a much softer sound. All of these designators are in reference to oneself or the primary noun of the sentence.
Chinese also designate siblings based on the order of birth. So it's not uncommon in more traditional families to hear them refer to, "this is first brother" or "this is third sister." The position, or pecking order, is important to traditional families as it referred to what level of authority and duties you had to perform for the family.
As one might guess from a fairly male dominated world, the first brother had the heaviest burden in terms of responsibilities and expectations. But they also had the greatest benefits normally in terms of education and opportunities. The first brother was expected to make a name for the family and carry on the lineage. Should something happen to the first brother, the natural line of progression would extend down based on the position in birth -- second, third, etc -- to assume the duties of the first son.
On the other side of the gender fence, the first sister often was the one looked at for helping to raise her younger siblings. In lieu of any mother or aunt, she would be the one to help run the household. Of course, this changes when she is either married out to another family or her first brother marries and his wife takes over the head position in the matriarchal side of the family.
Speaking of marriage, when a female marries, their "titles" only expand. They remain the sister position of their biological family. They also gain the new title, and a level of the responsibilities and pecking order, of their husband. So a daughter who was the first girl to be born and marries the second son of another family is both "first sister" and "wife to second brother." And when she bears a child, if that child is the first son to be born into the family, she could become "mother to first son," depending on what customs the family followed.
If this were not complicated enough, Chinese tend to believe that in some way everyone is "related" to everyone else. Thus as a sign of respect, one would often call friends or acquaintances that have no blood relation to them as their brother or sister. If the person is older than the primary noun, they would use the older versions of the term, and conversely the same with someone younger.
This actually isn't that uncommon from some more urban colloquialisms that have occurred in the United States, where people refer to those they feel akin to as their "bro" or "brother" and "sis" or "sister."
What all these separation of the simple terminology of siblings does is give the family structure and identity in just a few short introductions, something that is fairly critical in a traditional Chinese family.