I'm going to deny co-signing my brothers car loan
Why would you not help out your brother in his time of need?
He spends to much time surfing and playing on  his computer and needs more time to think of his future and get his act together.
I can understand your position. It is okay to refuse to be a co-signor on someone else's debt. You should explain how you feel to your brother and that you can't always help him out.
The problem is the car they are considering is a lemon. It's in poor mechanical running condition and it will likely soon require a lot of money in repairs.
You're right to deny cosigning his loan in this case. If you're on the hook for the money, you have a lot of say in what he does with the money. Why not tell him you'll cosign if you can accompany him to the car dealerships and have the ultimate say on the purchase?