What is the ruling on swearing an oath by divorce upon one's wife that she will not go to her sister's house, then allowing her to do so, what is its expiation, and is it forbidden to have intercourse with the wife after this oath?

The majority of scholars hold that swearing an oath by divorce (al-halif bi-al-talaq (definition)) has the ruling of a conditional divorce (al-talaq al-mu'allaq), and it takes effect upon the occurrence of the condition it was made dependent upon. This is the fatwa (definition) that is acted upon. So, if a husband swears by divorce that his wife will be divorced if she goes to her sister, and then she goes, the divorce takes place, even if he gave her permission, unless the oath was restricted by permission, or for a specific reason, or for a limited time. If the divorce takes place and this was the first or second divorce, then the husband having sexual intercourse with his wife during her waiting period ('iddah (definition)) is considered a return (rij'ah) to her. However, if it was the third divorce, she becomes unlawful for him until she marries another man.