Unblock dead grandfather's iPhone iCloud

Recently my grandfather died. Grandpa used IPhone 7 Plus. I got his phone. Now I want to give it to my sister because it is in good condition. Now I can't get out of iCloud, which has become blocked. I know both my username and password, but I can't sign out of iCloud. iCloud mail is also blocked to restore access. One security question I remember and the second I can not remember. Help me please. Thanks!

[Personal Information Edited by Moderator]