I think its important for spouses to stay out of arguments happening between their significant other and another family member. That is, if a man's wife is fighting with her sister, he should stay out of it, even if asked his opinion! Why the need for triangulation? The two people fighting need to resolve things between them without dragging in another party. As soon as you introduce another party into the argument, you have instantly TRIPLED the amount of possible animosity! Long story short, fight your own battles and don't seek out backup in the heat of the moment. Its ok to go home and ask for support, but don't escalate things in front of the other family member. That's just asking for long term trouble!