Is it permissible to kill a harmful cat that causes harm to my sister's children, especially after my sister's eldest daughter wet herself out of fear of being scratched by it?

If the cat is repeatedly harmful, it is advisable to persuade its owners of the necessity to curb its harm or get rid of it. If they do not respond, it is permissible to expel it. If its harm cannot be eliminated except by killing it, then it should be killed. The jurists have stipulated the permissibility of killing a harmful animal, such as a cat, but without torturing it; rather, by slaughtering it, in accordance with the Prophet's (peace be upon him) saying: "Indeed, Allah has prescribed excellence in every matter. So, if you kill, then kill well."