Put a stop to shouting and similar signs that an argument is spiraling out of control. A study published in "International Sociology" found that negative life events experienced by siblings are associated with increased closeness but also increased conflict as adults. Disagreements can be discussed by adults, but once a disagreement becomes a shouting match, it's difficult for either sister to hear what is being communicated by the other. If necessary, ask one sister to leave the room to cool down or suggest that you accompany one sister away from the site temporarily, such as for a coffee. If an argument gets particularly ugly, don't put yourself in danger. Use your voice and place yourself between the arguing sisters, if possible, to distract their focus.

Use skills for active listening in discussing the argument with adult sisters. Active listening is often used by counselors as an approach to psychotherapy. Skills vary, however. Texas A&M University recommends using "I" statements rather than pointing proverbial fingers to facilitate communication. Provide examples of active listening as ground rules for a discussion between arguing sisters and participate in the same manner. Listen without judgment and clarify what each person is stating, to be sure everyone understands what led to the argument.

Speak with each sister separately and act as an facilitator for conflict resolution. An document published by the Edmonds Community College explains that conflict resolution is meant to make communication more effective. This can be accomplished by encouraging both sisters to recognize, respond, forgive and compromise. Your role as the person mediating is to speak calmly and help both sisters calm down and begin the process of addressing and resolving the conflict that led to arguing. Keep in mind that if you are a family member, you may not be able to bring objectivity to conflict resolution. In this case, consider using a professional mediator such as a counselor.

Speak to the sisters' parents or spouses about the argument. Ideally, explains Forever Families, sibling rivalries that can lead to arguments decrease in severity and frequency once the siblings reach adulthood. For some siblings, however, old disagreements may continue, mainly because they were never resolved in the first place. Parents and spouses may be guilty of reinforcing the disagreement and subsequent animosity between sisters, making a bad situation even worse. If possible, speak directly with the parents as well as the respective spouses about the arguing between the sisters. Once they are made aware of the problem, parents and spouses can choose to intervene or not.