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For a damages claim of this amount, it is unlikely that this issue is going to go any further than it is already.
However, if the matter were actually pushed into small claims court (which it probably isn't), they could claim you were driving negligently (didn't see the obstruction in the path of your vehicle (regardless of where the cross bow was placed), and you could claim as a defense "contributory negligence" (the defendant’s liability will be offset by plaintiff’s percentage of liability. L.S.A. - C.C. Art. 2323) claiming that the brother's placement of his crossbow on the ground (again, irrespective of in the roadway or in the grass) was negligent (although if in the roadway, it would be an even greater percentage in your favor).
The end result is that while both of you would be liable for the loss, you would only owe him for a percentage of the claimed replacement damages. (Once again, I don't see this actually going to small claims court (it is simply too small a case), but if it did, the court would make a comparative finding of negligence, and you would owe him a percentage of the cost of replacement (replacement value is for the current value of the used bow, not for a new bow).