All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
When Satan incites a person to commit incest with one of his mahram (permanently unmarriageable) women, then he should seek refuge in Allah, the Exalted, and repel such evil thoughts. He should remember the punishment of Allah Almighty for such a hideous sin that causes His wrath. He should remind himself of the horridness of such an indecent act that is disdained even by many beasts, so how should it look to a Muslim whom Allah distinguished with a mind and blessed with guidance and faith?
Moreover, he should evade all that may arouse him such as gazing at them, being alone with them in seclusion, or the like. Allah, the Exalted, does not hold people accountable for their inner thoughts as long as they are not materialized into action. However, if he resolves to commit this sin but his efforts are hindered, then he is still deemed a sinner.
Allah knows best.