Is the brother-in-law's statement: "If your sister had stolen, I would go and divorce her" considered a promise of divorce, knowing that the wife had stolen when she was young and did not tell him, and the husband did not utter explicit words other than "divorce her"?

If the phrase mentioned is what the husband uttered, then it is a phrase not explicit in establishing divorce, nor is it a suspension of divorce upon theft; rather, it is a threat of divorce. Therefore, this phrase does not result in divorce, nor is the husband obliged to divorce his wife if he learns of her theft, because a promise of divorce does not take effect, and there is no obligation to fulfill it.