Is anything incumbent upon the questioner, who swore not to tell anyone what happened with another person, and then told her sister about it, without mentioning the name of the person involved in the story, but her sister recognized her?

Divulging a secret is forbidden, for the Prophet, peace and blessings be upon him, said: "When a man speaks a word, then turns around, it is a trust." You have broken your oath by telling your sister the secret, and you are required to offer expiation for breaking an oath. This is feeding ten poor people, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days, as Allah Almighty says: "Its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever finds not [the means to do so] - then fasting for three days." (Al-Ma'idah: 89).