Sorry I know this isn’t the right place to ask but idk what to do

So my sister’s kids are getting along really badly right now, after a fight they had a few months ago. And today, my older niece physically attacked the younger (they’re sisters) after the younger ate too many peaches (?) I’m not entirely sure how the fight started, but it seems that my sister was cutting peaches for them to share because my younger niece asked her, but apparently my younger niece was eating them really fast, so the older snatched the plate, then the younger took it back, and the older started beating her. Now she has this really bad bruise on her shin, a bit bigger than a golf ball. Idk how to put pictures, sorry. My older niece has a history of doing physically aggressive things like this before, but her parents have never really intervened. Her dad says that she’s too sensitive because she was crying, and my sister doesn’t want to hurt the older one. What can I do the intervene?