The .357 Sig is not very popular and hard to find ammo. It also kicks quite a bit more than the 9mm. I'd stay away from that caliber for what you need it for. A Glock in 9mm with quality ammo or perhaps .40 would be a good choice. A .38 revolver would also be a solid pick. One suggestion - take your wife to a shooting range where you can rent guns to shoot. Have her shoot a Glock and a .38 revolver and see which one feels more comfortable for her. Many ranges/gun shops will let you rent used guns to shoot for a small fee - it's well worth doing for something this important.

Also - the .40 can have a pretty good kick to it, depending on the gun style, grip, load used, etc. Best to try before you buy.