Is saying "I will kill you, kill you" in jest to one's sister, with an inner feeling that the intention was not to mock the Qur'an, considered disbelief (kufr) or mockery of the religion, and what should be done about this obsessive whisper (waswas)?

Jesting among Muslims should be regulated so that it does not deviate from truth, honesty, and Islamic manners. They must purify their tongues from anything that might cause Satan to sow discord among them, even in jest, for Allah Almighty says: "And tell My servants to say that which is best. Indeed, Satan incites [discord] among them. Indeed, Satan is ever, to man, a clear enemy." The Prophet, peace and blessings be upon him, warned against the harvests of tongues. Therefore, these words must be avoided, even if said in jest. This phrasing is not considered mockery of the Quran but rather falls under the category of obsessive whisperings, which should be disregarded.