You can't stop them. The best you can do for yourself is learn how to maintain your independence from them and keep yourself out of their line of fire as much as possible.
First, you should read up on the disorder. There are dozens of good websites sponsored by well known hospitals and other respected institutions that sponsor web pages devoted to Narcissism and narcissistic personality disorder. Narcissistic behavior is predictable. If you have a good understanding of narcissism you will have the tools to deal with it more constructively. Here are a few pointers:
Either use duct tape or cut contact. The narcissist will not change his/her ways.
You are saying at what age do they stop showing signs of narcissism but they can't because you are saying they are not really a narcissist so how can they!
yes cause they can call collect
To make a sibling stop crying, you must first find out why they are crying. Are they hurt or upset? Are they looking attention? As soon as you can deteremine the reason for their crying, the sooner you can get them to stop.
Get a police order of protection
Talk with your parents and ask them some suggestions. Help your sibling make friends, besides yours, and get some interests.
give her a slap in the face
No
As many times as the supply will allow him to. So STOP allowing him to.
No. A narcissist cannot change. Narcissism is an untreatable personality disorder. It is unrealistic to believe a narcissist can change any behavior. In the world of the narcissist, he or she is center, remorseless, and sees no reason to change. He may manipulate his chosen one into believing he will change as manipulation and omnipotence are core to his insatiable need to control and keep his partner. Losing is not an option for the narcissist. Because of his inability to feel remorse, the narcissist acts out normal responses based purely on observation of others. He may say he will stop drinking to keep the relationship of abuse and chaos intact. Indeed, he cannot perceive a reason not to do exactly as he chooses to do. The narcissist lacks the ability to be accountable to any person or promise. He has no sense of remorse, no need or feelings for others exept to objectify
The easiest way is to treat them both equally. Try taking them out one on one, about once a month. That might help. I think that sibling rivalry is usually rooted in jealousy.
There is every hope. Don't ever allow them to spend ANY time alone with the narcissist and protect them from their criticisms and reaffirm their self worth and value if the Narcissist gets to them. if things get bad, just stop taking the children. Their emotional, psychological and mental protection is paramount.