Can the evil eye be a cause of a person's death, and was what happened to my sister's fiancé due to the evil eye, or is his death unrelated to that? And could his death be a punishment for her for not praising and thanking Allah for the blessing of marriage and for not being content with her destiny? And what are the verses for ruqyah against the evil eye and envy?

There is no definitive reason for what you mentioned, and it is not permissible to accuse someone afflicted by a calamity of not praising and thanking their Lord. As for the evil eye, it can indeed lead to death, as stated in the Hadith: "The evil eye can put a man into his grave and a camel into a cooking pot." It can also lead to sudden and severe illness, as happened to Sahl ibn Hunaif, may Allah be pleased with him. For protection against the evil eye, one can refer to the mentioned fatwas.