To start this off, I will at least say that if you do not have a sibling, I feel for you.
Having someone in your family like that is extremely important. Perhaps you may not have siblings, but you have a cousin, best friend, or another family member that helps this relate to you. For me, however, this is all about my sibling.
I have one brother. I am currently 19 and my brother is 22.
Growing up, we never were close. As siblings normally do, we fought a lot. We would constantly call one another names, and he made me cry every so often. As we got older, sometimes we would get along for a few minutes. We never saw much of each other with our parents being divorced and being in different schools. We went to the same school for three years until the school system changed, and then only one year when he was a senior in high school and I was a freshman. Besides that, we did our own things. He would play video games and I would hangout with friends. Every once in a while we would talk and maybe ask each for help with a homework question or chores, but most of our communication in high school consisted of tired grunts in the morning to use the bathroom. We continued this until our mom had a stroke and we needed to band together to take care of her. We knew that she needed us to get long so we did.
Needless to say, our relationship took a turn when our mom passed away. This happened about a year and a half after her stroke. We had been getting along pretty well, but when everything happened we knew that we needed one another. We were the only two in our families who could understand each other. We suddenly stopped fighting. Our worlds came crashing down and we only had one another to hold on to. This sounds dramatic, but that's because it was. We literally had no one else to turn to for a better understanding.
Our mom passed away almost 2 years ago. To this day, we are still best friends. Without my brother, I do not know where I would be. I appreciate our 2 AM talks, our twitter 'arguments', our bickering, and our Taco Bell runs. We hangout like best friends do, but we live in the same house. He likes to tell me we aren't related when I do something weird, but I know deep down (deep deep down) he loves me. When people say that your sibling is the most important person you can have in your life, they are not lying to you. It is the truth. I understand that sometimes siblings fight, and it can be difficult to support them. In the end, though, they are the one person who will be there for you.
Remember this next time you and your sibling argue. Remember that no matter what, they will be there. They would take a bullet for you. They are your blood and you are theirs. Family is family, no matter what.