Polynices and Eteocles fought each other because Polynices fought for the army that went against the Thebes. They both fought to the death. Only Eteocles was able to be buried because he wasn't a traitor like Polynices.
Antigone and Ismene talk about burring their brother, Polynices. But only Antigone wanted to go through with the plan since they could be killed if they were caught burring a traitor.
The Chorus knew that there was going to be a traitor who fought against the Thebes. This being Polynices, Antigone's brother who she wants to bury.