Listen to Them
It is best to listen to them and understand that what kind of issue they are facing with each other. Often the issues are petty in nature. They just let the frustration that is bottled inside them out at each other over small things.
Talk to Them
Talk to them about the importance of respect as well as love for each other. Do remind them of the times when they have had so much fun and let them know that there is no reason for it to be any different now. They need to keep their space but respect each other as well.
Remove the Issues
Often the bone of contention could be on the amount of computer that one has to use or something similar in nature. Remove the issues tactfully so that there is a solution that works for all.
Treat Them as Adults
One way to go about it is to treat them like adults and they will certainly act in a much more mature manner. Teenagers have great mental abilities and have developed a fair bit of sensibility. It is important that the abilities are put to use so that the energy that they have is given a positive outlet.
Give them tasks in which they can bring their energies to use and they will not only have a way to make use of abilities but also will gain confidence knowing that they can perform the task.
Make Them Do Activities Together
Doing activities together can be an excellent way to improve a relationship. Make the siblings do house chores and other activities together. This will help them in having respect for each other. They will also be able to connect better which is often lacking because they do not spend enough time together.