Meddling sister...

It is okay to meddle if it is already about physical abuse but if in case it is just a minor problem that the couple could sit and talk in order to solve it, then it should be left between the couple alone the sister should no longer have the right to meddle with it.

Her neutrality might be at question if the in-law doesn't agree with her conclusion as to who's to blame and how to settle the fight. Couples though who are fighting always want somebody to mediate between them but most times that person becomes the subject of their disdain when they settle. So the sister might not like it afterwards.

I think it shouldn't be. However, it depends on the situation. For me, it is better to walk away first. Then, he or she can go back after.