What is the ruling of Islamic law on a man marrying his maternal aunt's daughter without knowing that she is afflicted with a jinn, and what should he do after finding this out and before consummating the marriage, while taking into account the ties of kinship?

A word of advice is to strive to treat your cousin who is afflicted by jinn possession with legitimate ruqyah (definition), and to be patient with her if possible. However, it is permissible to request the annulment of the marriage and the return of the dowry if the jinn possession reaches the level of epilepsy, which is considered a form of insanity and prevents enjoyment [of marital relations], or if you are unable to remain with her in this state. This is not considered severing ties of kinship. It is preferable to marry a second wife while still treating her kindly, if possible.