Will the daughter be sinful for not believing in her mother’s death, despite her faith in Allah’s divine decree and destiny, and will the mother disown her for that?

Death is a decree from Allah upon every soul. It is recommended to be patient and to say "Innā lillāhi wa innā ilayhi rājiʿūn" (To Allah we belong and to Him is our return) when afflicted with calamities. Allah the Almighty gives glad tidings to the patient, bestows His mercy upon them, and rewards them with great recompense, even for a thorn that pricks a Muslim. The entire affair of the believer is good: if prosperity befalls him, he gives thanks, and if adversity befalls him, he is patient, and both are good for him. When a calamity strikes, one should remember the calamity of the Messenger of Allah, peace be upon him, for it is the greatest of calamities. Allah has promised Paradise to the believer who seeks recompense from Him for his lost one.

As the Prophet, peace be upon him, taught us to say: "Verily, to Allah belongs what He takes, and to Him belongs what He gives, and everything with Him has an appointed term, so be patient and seek recompense." And to supplicate: "O Allah, reward me in my calamity and replace it with something better." The pain of loss you feel will subside, and there is no sin upon you as long as you guard your tongue and limbs. We advise you to fear Allah, pray for your mother, and be good to her.