بسم الله الرحمن الرحيم
(Fatwa: 319/316/B=04/1436)
You Nana was an Islamic scholar as well as your teacher. Also he was the real father of your mother. It was regrettable that you spoke bad about him that too before your mother. You hurt your mother either. First you should seek pardon from your mother and then do true and genuine taubah. Now you should do du’a for your Nana’s forgiveness as well for his high status (in Jannah). Recite the holy Quran etc daily and send its reward to your Nana continuously. It may suffice as kaffarah.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband