Is divorce effective by the saying, "My wife is divorced if you don't say it," if it is highly probable that she did say it?

If your intention in making your wife's divorce contingent was on whether your stepmother actually uttered that word or not, then the divorce takes effect if your stepmother denied saying it, unless there are trustworthy witnesses who testify that she did say it. However, if your intention was to affirm the occurrence of the speech based on your dominant assumption, then the preponderant view is that the divorce does not take effect. It is preferable to refer the matter to the Sharia (definition) court.