Zip codes do not matter for life insurance premiums. The factors used to determine a life insurance policy do not include location. Your address and zip code are a factor when it comes to home insurance and auto insurance but for life insurance it does not matter.
Life insurance is affected by these factors: individual health factors, family health factors, smoking, age, gender, health conditions, lifestyle, hobbies, occupation, and travel. The biggest things that affect a life insurance policy are your personal risks. If you are a smoker that puts you at a higher personal risk than a non-smoker. Duration also affects a life insurance policy. The shorter the duration the higher the premium. This is why the older you get the more expensive you health insurance will be.
As far as zip code goes your address only affects where they will send your documents. It will not have a bearing on the policy itself. As you are paying for your brother's premium you would most likely want info pertaining to that premium delivered to your house. However, I would check first if that is okay. Most times it is as companies allow clients to have two addresses, a personal one and one for their mail.