Thanks to all for your comments ! Seems like I was right to suggest it to my brother
Motan, thanks a lot for all this information ! I will take pictures tomorrow to illustrate what I'm about to say.
I do think that the sheath is original to the knife for the following reasons : the material (brass alloy) on the hilt and the sheath are indistinguishable, the hilt slides into the scabbard (approx. 1cm) with a very snug fit, and the border with a zigzag pattern surrounding the central motifs is similar on the sheath and the hilt.
The blade indeed does seem almost Indian inspired (chilanum ?), though I believe it was made in the Levant, as it looks good from a distance, but exhibits rather simple craftsmanship upon closer examination (especially at the end of the fullers).
Anyway, I will try to take details pictures tomorrow. Thanks a lot for your help, and to your friend Mahmoud for the translation ! I'm glad to put a name on who made this beautiful knife !