First, let's identify the save game directory, find this path:
In this directory you can find the save game named:

Let's say that this is your save game and you want to let your brother play Dealer's Life without messing with your playthrough, you can copy and rename the save game to remember that it's your save, for example "savegame_Tanek.dat".
When you start the game, you can let your brother play and create a new save game (which will override the existing one) because you have backed it up.
When your brother ends playing, wait for the end of the Steam Cloud synchronization (you don't want to mess with files while it is saving on the cloud!) then copy your brother save game and rename it to remember that it's your brother's save, for example "savegame_Fiz.dat".

Now, each time someone wants to play, you have to overwrite "savegame.dat" with your or your brother own save (always copy it, so that you are covered from mistakes) so that the game finds the your or your brother save game to continue the correct one.
And each time you or your brother end playing you must update the backup, for example "savegame_Tanek.dat", to keep it up to date!

Please be careful with these operations, if you are not sure ask us more details!