Welcome to the forum.
Most of my factory made guns were made before 1997 and they all are totally safe to shoot. Your brothers gun should be equally safe.
Although the modern synthetic black powders like Pyrodex do weaken a bit over time, if the container was tightly sealed it should work without any problems.
If you do hunt with this gun, you might want to buy some newer powder just to make sure it has its original power.
If the powder is real black powder and it was kept dry it should be as good as the day it was made. Black powder doesn't lose its power over the years.
Be sure to get all of the things that look like they might be used with a muzzleloading rifle, even if you don't know what it is or what it does. If it really is something for your rifle, we will be able to tell you exactly what you have and what it is used for.
Don't be surprised if you become thoroughly "hooked" after you've shot a few rounds. These things are great fun.
The one thing that is very important is all of these black powder guns need to be cleaned as soon as possible after your done shooting.
The fouling left by the burned powder will absorb moisture out of the air and it will rust the bore in short order.
Use plain water to clean the gun and apply a light coating of a good oil to prevent rusting.
Have fun. :grin:
PS: Never shoot any kind of "smokeless powder" in your gun.
Real black powder, Pyrodex, Seven7seven or any of the other replacements that say they are for black powder shooting will work if your gun has a "cap lock" that uses percussion caps.