How can I play Civilization V multiplayer with one copy

I'll give you a couple solutions that fulfill the spirit of what Steam allows for sharing a single copy of Civ V.

1. Simple Hotseat mode: You play on a single computer playing a game of musical chair. Passwords for each player help minimize the chance of information bleed between turns. This technique is a bit faster and has the added bonus of built-in calisthenics.

2. Giant Multiplayer Robot: Think of it as multi-machine or even multi-time-zone Hotseat. You each download the GMR client. When it's your turn it will download the save, and launch Civ V for you. The client can also be configured to automatically close Civ V when you save at the end of your turn, so that your brother can use it. With in-home-streaming configured, you don't even need Civ installed on the 2nd machine. Family Sharing may also need to be set up between n+1 accounts; Owner (Steam) + n Player (Steam & GMR) accounts. I haven't tested all of the details of this as all my friends have their own copies of Civ V. While slightly slower, this solution allows you to ask for advice here while your brother is taking his turn. ;)

I can't think of any other solution that would not put your Steam Account(s) (and any attached games) at risk...

Another solution would be using the family mode steam provides.

Civilisation V still provides a "LAN" multiplayer.

All you would have to do after having setup the family mode on your brother's computer, would be to launch steam offline on one of your computers, launch Civ 5 and create a LAN game.

I'm not certain this will work since Civilisation could rely on another layer of protection to prevent two people with the same license accessing the same game.

I can not assure you that it is fully compatible with Steam TOS since the family mode should only allow one people to use the shared library at one time (the main owner of the game always getting priority).

A single copy of Civilization 5 purchased on steam can be used on multiple computers within the same local network (LAN) without too much drama. Another answer mentions family mode but it's optional.

After you installed steam and the game on all machines within your LAN, make sure that:

• 1. You specified different Nick Name in the Local Network section of Multiplayer Options on each computer you would like to play the game on together
• 2. You have correct Firewall settings. I suggest that for testing purposes just temporarily shutdown you Firewall, you can fine tune the firewall rules later
• 3. Put steam in the offline mode. Offline mode will prevent Internet Games, but Local Network Games are supported in the Offline mode
• 4. If you have more than one network adapter with different IP addresses, which is relative rare, but does happen, e.g. in case of VPNs or wired and wireless connection at the same time, the game just picks up one. If it's the wrong one, that is if it's not one your LAN is running on this will not work. Disable all the network adapters but the one, that you are using for your connection

After this is done, use Multiplayer, Standard, Local Network to host the game on one machine, and to join on others.

Note: you mentioned that you and your brother are connected with WiFi, in some rare cases WiFi could have client isolation feature turned on, which would prevent different devices on that network to talk to each other. Make sure that this feature is not enabled on your WiFi

This is where you set up your nick, different on each machine:

This is the top left of your Steam window, where you go to offline mode

This is your Windows 10 Network connections Window. You will have a different set of adapters there. Search for (type) "Network Connections" in the start menu. Note how everything but your main LAN connection is disabled.

This is assuming that you are using Windows Defender firewall and not a different Firewall. You might need to find instructions for your own Firewall in order to temporarily disable it.