What should I do about a relationship with a married woman that developed into sexual phone calls and threats, leading to her illness, and what is my sin in this matter, and how can I rid myself of this sin for fear of God's punishment?

Chasing women via Facebook and chatting with them is a great door to fitna (temptation/strife), especially for a married man who is able to keep himself chaste through halal (lawful) means. Opening the door to sins leads to dire consequences; in addition to chatting with a married woman and forming an emotional relationship with her, a person falls into "incitement to discord" if he talks to her about marrying her after she divorces, which may push her to seek a divorce. Also, threatening her to use her as a means of blackmail is forbidden in Sharia (Islamic law) and may cause her illness. Therefore, sincere repentance is obligatory, along with striving for good in the future, increasing acts of obedience, and supplicating and seeking forgiveness for this woman. It is not necessarily true that what happened will result in punishment for one's family and children. It is recommended to frequently recite supplications asking Allah for well-being.