What is the ruling on one who went to a diviner, asked her a question, and believed her, while justifying that he was seeking refuge with Allah from Satan and disbelieving the astrologers?

The connection between playing cards and visions or soothsaying is not clear. There is no harm in your cousin seeing a vision of you and your hearing about it from her, as long as there is no impermissible mixing (of genders), for she is not your unmarriageable kin (mahram). If your cousin's actions involve claiming knowledge of the unseen (ghayb), then it is not permissible to go to her or believe her, due to the Prophet, peace and blessings be upon him, saying: "Whoever comes to a soothsayer or a diviner and believes what he says has disbelieved in what was revealed to Muhammad, peace and blessings be upon him," and his saying: "Whoever comes to a soothsayer and believes what he says has disavowed what Allah revealed to Muhammad, peace and blessings be upon him; and whoever comes to him without believing him, his prayer will not be accepted for forty days." The door of repentance is open.