Did he just pressed "start a new game" or played for some time?
If he didn't passed further than De Pleur's torture basement, may be one out of two (or more) your older saves is still in FC4 saves folder on your computer.
Try to do the following:
1. Turn cloud synchronization off (uncheck the box near the option "Enable Cloud Save Synchronization.." in Uplay general settings).
2. Locate your FC4 saves folder ("420" or "856" or something else, read the instructions
here) and back it up in a different place.
3. Open this folder - you will see a set of numbered pairs of
*.save and
*.save.upload files (sometimes it's just
*.save files). Pick only one of these pairs/files and delete all the rest. Obviously, you want to restore your older save before your brother started a new game, so you should delete the most recent one. Now, if the number in the file/s name/s is different from "1", rename it to "1".
4. Load the game, and see if you can continue your previous game. If it didn't work and you have more then two saves, exit the game, restore the content of FC4 saves folder from the backup you made in step 2, and try to leave different older save file/pair as described in step 3.
In the future I recommend to turn cloud synchronization off permanently and instead regularly backup your saves as described in my guide. You can create two separate folders for each brother.
If you would just backup your save before clicking "start a new game" in the first place all this grief would be saved...
There is also a possibility to recover some of your older saves through general system restore to a previous restore point.
You can also take one of my saves. The last one is at 87%, 3 campaign missions left, 1 outpost and 1 fortress left to liberate, 2 Shangri-La missions and Yak farm.
https://dl.dropboxusercontent.com/u/...ollection2.rar