Yaar incest kahani aur filmon ke plot ke sath ho tou banda usko plot samajh ke parh ya dekh leta hai. For example game of thrones. Lekin agar har cheez mein incest hi dhoondo ge tou dimaag tou chakraye ga hi. Jaisa ke yahan aksar readers ke saath hota hai. Aisa nahi hai ke mein koi bhi incest wali story nahi parhta but apna fanda simple hai kahani parho time pass ke liye aur bore ho jao tou break it off and walk away. Zyada serious lene ki zaroorat nahi hai. Simple formula hai, 10-15 update story ke parho agar sex ke ilawa kuch na mile tou bhaag jao. Aisi story aapka dimaag kharab kar degi.
And to answer your question, it's not normal to fantasize about your mother. Psychiatrist ko dikhao man...