Has your brother had a hard time finding .357 Sig ammo? I can't find it locally. I don't have one but one of my co-workers does so I'm always scouting for ammo for him.
The .357 Sig is one heck of a round, but It's still not that common. Some find the recoil to too harsh, I don't think it's that bad, but what it is is SUPER LOUD. I'm not going to debate which caliber is more effective as I fall into the camp that thinks if you put the bullet where it needs to go, there isn't much difference between the major service calibers. Where the Sig round does excel is when going through cars to address a target inside the passenger compartment. I just posted this link in another thread and you can see some of the data regarding the .357 Sig and others.
If you plan on buying your brothers, I'd say go for it, if you don't like it you can put a .40 barrel in it and call it a day.