Cops won't do anything, trust me. I'm not sure what would happen since their both minors, but they would pretty much say that because HE'S older - he should know better (e.g. walk away, lock yourself in room). And they would tell him that's it's their parent's responsibility to handle his brother, not him.
Plus, cops don't give a shit what happened if you don't call while it's happening. I suppose you could try this, but honestly it's not a nice situation because if things escalate - it'll all fall on your parents (paying for lawyers, stress).
And pouring soft drink over each other probably isn't a life threatening situation either (but if it's escalating keep my above post in mind).