I'm confused. (Spoilers)

It did happen in the midst of a celebration, and could have initially gone unnoticed. More likely the explanation is that it's not honorable to intervene in a fight unless you've got a dog in it. It wasn't immediately obvious that weapons were involved...began as a fair fight with rough housing and skin on skin. By the time weapons became involved, it was too ate.

(And I believe Vance was the kind of man who'd let his family die if it served his own purpose.)

Ellison had a line where he insisted that no other McCoy get involved, so they were respecting his wishes, as long as they saw it as nothing more than a fist fight. I assumed because of how fast everything happened, and the stabbings were at close range, nobody noticed.

Ellison had the fight pretty well in hand. The McCoy brother that pulled the knife out only did so after being knocked a down and out of the fight for a moment. All eyes were still on Ellison and the other two McCoys when he pulled his knife. He then concealed it in his fist in such a way that as he was stabbing Ellison it appeared to be merely kidney punches...nohing to be too concerned about given that Ellison still appeared to be doing well.

It wasn't until the gun was pulled that people even suspected trouble.

I thought it was impossible NOT to notice the multiple stabbing.
Therefore I was quite mad towards those two Hatfield idiots standing by doing absolutely nothing. Poor Allison.

Boycott movies that involve real animal violence! (and their directors too)