The top of the slide in the .357 SIG and .40 are wider than the 9mm.
I have two holsters made for P239s. When I still had the .357 SIG I would use it interchangeably with the 9mm in both holsters. Since the real retention is at the trigger guard it was not a problem. There was a minor difference in the effort needed to draw the pistol. The 9mm was a little easier than the .357. You would notice if you tried that the 9mm fit the holster a wee bit more loosely than the .357.