Is there sin involved in having sexual relations with one's wife after swearing not to have sexual relations with her and likening her to a sister as a threat, with no intention of divorce?

If you swore not to have intercourse with your wife until she is pleased, and you did not do so, then there is nothing upon you. Your statement to your wife, "You are like my sister until I am pleased," is considered a temporary dhikr (a pre-Islamic divorce formula comparing one's wife to an unmarriageable relative), if you intended dhikr by it, or if a strong indication like anger pointed to it. If you have intercourse with her before she is pleased, the expiation for dhikr becomes obligatory upon you. However, if you have intercourse with her after she is pleased, then there is nothing upon you. You must repent and seek forgiveness for this statement.