Are involuntary sexual fantasies, which extend to family members, considered a sin, and what is the way to get rid of them and remain steadfast in the religion?

Praise be to Allah that you have repented from watching forbidden movies. This is a blessing that necessitates gratitude and preservation through steadfastness. Satan intended to corrupt your repentance and your religion with these thoughts, which will not harm you if you repel them and are not affected by them. However, if they settle in the heart and the soul takes pleasure in them, then you will be held accountable for them. So, beware of these thoughts, renew your repentance, frequent the mosques, recite the Quran, accompany the righteous, and remember standing before Allah.