Will a person be held accountable for sexual fantasies that involve intercourse between a person and a relative, followed by the release of desire based on this fantasy?

Sexual fantasies between a man and a woman forbidden to him are impermissible. If such fantasies become ingrained in the heart, this constitutes a form of metaphorical zina (fornication/adultery) and can be a means to falling into actual zina. A Muslim should occupy himself with what benefits him and be diligent in guarding his thoughts, lest they lead him to what has dire consequences. Marriage is one of the best ways for a Muslim to protect himself, and one should hasten to it as much as possible.