Most siblings get over their sibling rivalry by adulthood but some brothers and sisters continue to fight after this point.
In the Bible, in the Book of 1 Corinthians, chapter 13, verse 4, we are told that love is patient and kind. Siblings need to be patient and kind with each other. If you are having problems with a younger sibling, try to remember what it was like to be their age when you didn't know as much, couldn't do as much and weren't as mature as you are now. If you are having problems with an older sibling, remember that since they're older, they are experiencing new things that you haven't yet experienced yourself. Thus, they have more on their minds than you do.
Many times when a group of siblings fight, the problem rests with the order in the home. Jesus Christ tells us clearly in the Bible that he is to be in charge, or the head of every man. The man is then to be in charge of the home with his wife under him. The children are then supposed to be under the husband and wife, their mother and father, of course. Oftentimes when there is intense fighting among siblings, God's order is not in that household.
1 Corinthians 13 4 also says basically that love doesn't put itself above another person and is not puffed up. If you are really good at something and get lots of attention for it remember that your sibling or siblings could feel left out or neglected.
If you are the sibling feeling left out or neglected, remember this. God created each person individually with his or her own gifts, talents and abilities. Just because your sibling is really good at something doesn't mean you're no good at anything. Try to find out what you're good at and, if you already know the areas where your talent lies, take time to become the best you can be in whatever area you do well in.