Pummel Party, Stick Fight, Space Marine 2, For the King, Borderlands are would be my recommendations.

You will need games that have crossplay between PC and Xbox One so you can play together across different platforms. Multiple people have suggested Left 4 Dead 2, but that doesn’t have crossplay. Most of the shooters I personally play don’t have it, but there definitely are shooters with crossplay.

Here are my recommendations that have crossplay:

• Borderlands 3 — Collect wacky guns, travel the universe, and shoot bad guys together. The previous games in the series don’t have crossplay.

…Yeah, that’s it for crossplay shooters I recommend. For crossplay games that aren’t shooters:

• Overcooked! All You Can Eat — Chaotic co-op cooking. Work together to prepare, cook, and serve food in increasingly absurd scenarios: in the middle of the highway, on an iceberg, in a hot air balloon that crashes into a different restaurant.
• Ultimate Chicken Horse — Platformer where you build the levels together and then race to the finish. You only score if someone died, so you need to make the level extra dangerous.
• Moving Out 2 — Goofy co-op game where your group plays as a ridiculously reckless moving company. Carry furniture from the house and shove or throw it into the truck. No one will notice if you break all the windows.

If everyone is on PC, things will open up a good bunch. Old-school networked games generally still work. You can go FFA deathmatch in your old favourites or in newer arena shooters, like Warsow or Disco Dodgeball.

Another vote for deep Rock galactic.

Worms?

Halo Infinite has free multiplayer that is cross-platform between PC and Xbox, and works on the Steam Deck. There are lots of competitive and co-op modes available, both in official playlists and community made Forge modes.

You can make private matches until you’re comfortable joining public ones.

The campaign is also online co-op, but must be paid for.

First thing that comes to mind is Warframe. It’s a co-op third person looter-shooter, with full crossplay, so you can all party up across your platforms. It’s all very controller friendly, with lots of shotguns, SMGs, melee weapons and space magic that are all really forgiving of imprecise aim. It cares less about twitch reflexes and more about movement.

The scifi setting and “space ninja” aesthetic may or may not be to your taste, although I promise if you take the time really sink into the world it’s actually one of the most refreshingly different and unique scifi settings out there. There’s a lot of weirdness, but as you dig deeper into the story that weirdness all makes sense. And, like, it’s the good kind of weird if you get me? Stuff that makes you go “Holy fuck I want to know what the deal with that is!”

It does have a lot of MMO elements, so it can get grindy at times, but in my experience it’s a really solid game for hanging out and chilling on Discord together. Plus the game itself is free, with no paid DLC or add-ons, and for an adult with an income a few bucks here and there skips a LOT of grind, especially if you check out the third party market website where players will sell you a lot of the rare drops you’ll want for less than a dollar.

Added bonus, it’s made by the original developers of Unreal Tournament, Digital Extremes (there are actually a bunch of UT references squirelled away in the game).

The Borderlands series might work. Also I believe L4D2 might run on all those, if y’all wanna coop some zombies?

If not for the fact you said one’s on an Xbox, for a co-op experience I’d recommend Helldivers 2. However, a similar game to HD2 is Deep Rock Galactic, and it’s on Xbox as well as PC. Fantastic co-op shooter with some very funny humor, much like HD2 you can tell a lot of love went into the game; and much like HD2 the developers actually give a shit about it and their community. Since you said you don’t have a load of screen time, you should know a full mission is usually about 20-30 minutes.

For a PVP experience, you said you have two brothers – three is actually the perfect number if you wanted to get Sniper Elite 5 and play through the campaign with Axis Invasion enabled, you can invite one to play co-op and for the other, you can invite a specific invader. Two players would be playing as allied snipers infiltrating various locations in France and taking out Nazis, and the third would be an elite German sniper hunting them. It’s supremely fun to hunt people (as well as be “hunted” – though with two players you can really flip that on its head). And a mission in SE5, depending on how you play, can either take as short as 15 minutes or as long as 45 minutes, though probably on the longer end if you’re doing invasion – you do not play it like COD if you actually want to survive against a hunter.

Both of those have crossplay and should work on a Steam Deck, as well; although I’ve noticed Sniper Elite 5’s anti-cheat, when run through Proton on Linux, doesn’t like the game being installed on an external drive and won’t let you play online if it’s not running on the same drive the OS is installed on, and I imagine that’d probably extend to SD cards. So, if you go for SE5, make sure that brother installs it to the Deck’s internal storage or they might run into trouble.

Going to go out of the shooter box here and say Valheim. I am pretty sure it’s cross compatible with xbox and pc/steam deck. Might not be the thing though if you are just all wanting to pvp, but thought I would throw it out there.

Civ 5 was great for this. Me and another old fart in our 40s played it against each other.

Portal 2 is only two player, but it’s my favorite game and it has a co-op mode with puzzles that require both people.

Enshrouded, Chained Together, Payday 3, Risk of Rain 2, Helldivers 2

I’m a huge fan of Borderlands so would recommend that and I’m also a huge fan of Halo, the Master Cheif Collection is an absolute bargain considering what you get. Your brother with the Xbox will need to get Game Pass of some sort to join in for any games that are not free to play. Halo Infinite is free to play and has a PvE game mode, if that tickles your fancy.

Palworld is another kind of game playable multiplayer on xbox/pc.

Here’s a full list of crossplay Xbox and PC games. I figure if it’s on the pc, there’s a good chance it could be played on the steam deck as well.

Notable mentions in there that I’ve played and enjoyed are Destiny 2, Overwatch 2, Remnant 2, Left4Dead 2, (I’m seeing a pattern here), and Deep Rock Galactic.