You may have an easier time dealing with your sister-in-law if you understand possible reasons for her narcissistic characteristics. Some experts believe that narcissistic behavior is rooted in childhood experiences, according to Randi Kreger, author of the article, "Shame Is at the Root of Narcissistic, Borderline Personality Disorders," published in Psychology Today. The article explains that some research shows parental deprivation and constant shaming may lead to narcissism. Other researchers believe parents can do too much for their children, causing them to feel entitled and become narcissistic. Ask your husband about his sister's childhood and try to feel empathy toward her.

If you find that dealing with your husband's sister always results in your blood pressure rising, you should set some limits. It is not necessary to explain your boundaries to your sister-in-law. For example, if you find helping prepare family meals with her stressful, you might go set the table or offer to clean up afterwards instead. Explain to your husband that you need to limit your interactions with his sister in certain situations to avoid problems.

When you find that you and your sister-in-law have a difference of opinion, avoid trying to reason or argue with her. It is best to clearly and briefly state your position and avoid getting into a discussion. If she tries to instigate an argument, you should take the high road and refuse to engage. If your disagreement is about something important, such as treatment of your children, enlist your husband's support and stand your ground.

You may be able to build your relationship with your husband's sister with practical displays of support. Maybe you can help her with tasks such as cooking or household chores to show your support. To prevent her from stepping in and taking control of your efforts, help her with something that is outside her expertise. Your attempts to be closer to your sister-in-law will show your husband that you value his family.