Does divorce occur if I get to know my sister again after I swore by divorce that I would not get to know her again, or is there an expiation?

If you broke your oath by doing what you swore to refrain from, your wife would be divorced, according to the majority of scholars, and you could take her back before her iddah (waiting period) expires. Some scholars, like Ibn Taymiyyah, hold that if you did not intend to divorce your wife, but rather intended to emphasize or threaten with the oath, then no divorce occurs; instead, you are required to offer an expiation for breaking an oath (kaffarat yamin). Severing ties of kinship is forbidden, and maintaining them is obligatory. The Permanent Committee for Issuing Fatwas has ruled that entering the nephew's house is obligatory as a way of maintaining kinship ties. If the intention behind the divorce oath was to prevent yourself from entering, not to divorce the wife, then an expiation for breaking an oath is due. However, if the intention was to divorce the wife, then it is considered a single divorce, and the wife can be taken back as long as she is within her iddah, provided it is not the final divorce.