Do you happen to know if your brother played at all with other people, or just collected? If he played, perhaps there are other players locally who can help you go through it in person, so it's not as such a daunting task.
If you want to sell it all in bulk just to be rid of it, very few people would blindly buy a lot for any significant amount of money. But if that's your primary objective, just be aware that's the trade-off.
If you'd like to sell them to make some money, the first step is to sort and organize them and inventory what there is. Anyone you're going to sell them to will want to know what they're buying. And you want to know what you're selling. If there's a
James T. Kirk card, the Ultra Rare from The Motion Pictures expansion, that's worth about $500 currently in mint/near mint condition. If it's mostly First Edition Premiere, Unlimited/White Border with 1995 copyrights, that's the least valuable cards there are because there's so many.
If you do manage to inventory it, you then have a couple of different options. Sell the entire collection in bulk to someone who will likely resell it. CategoryOneGames.com does buy collections, depending on what they need at the moment. But they resell cards, so you won't be getting full "street value". But then you've made some cash and it's done with.
If you sell directly to collectors and players, you can maximize your revenue for the in-demand cards that people want to buy. But you may be stuck with the common stuff no one cares about. At that point you can trash it, recycle it, donate it to Goodwill (they do online auctions of donated card lots), or give it away online.
You also want to pay attention to the condition the cards are in. If they're all in binders and have been protected over the years, they should sell well. If they are bent, the edges frayed, water or smoke damaged, they will be worth significantly less if anything at all.
This understandably seems like a daunting task. But if you identify your main goals and set a plan, and take some time to go through it (even a few hours a week), it is something that can be done.