Does a divorce occur by saying, "And if I touch her after today, then our marriage is void," without the intention of divorce, followed by touching the wife's sister twice with lust? And how many divorces occur if a divorce takes place?

Firstly: Touching the wife's sister with lust is forbidden and constitutes the fornication of the hand, about which the Messenger of Allah, peace and blessings be upon him, informed, for he said: "Indeed, Allah has decreed for the son of Adam his share of zina (fornication/adultery), which he will inevitably attain. The zina of the eyes is gazing, the zina of the tongue is speaking, and the soul yearns and desires, while the private parts either confirm or deny all of that." The Messenger of Allah, peace and blessings be upon him, warned against touching a non-mahram woman, saying: "For one of you to be stabbed in the head with an iron needle is better for him than that he should touch a woman who is not lawful for him." It is obligatory upon you to seek forgiveness, repent, and resolve not to repeat this, and not to be alone with her or speak to her.

Secondly: Your statement to your wife, "If I touch your sister, then our marriage is void," is one of the allusions to divorce. Divorce does not occur with it except by intention. Since you did not intend divorce, it does not occur.