I have used the translation "Ahéheeʼ shimasaní.", and it was rejected. I have made a report to be added.
Sentence suggested by Duolingo:
Hints suggested for "grandmother":
The translation "shí nalí asdzaan" is lacking. Reported.
Shinálí = (my) Paternal Grandmother
Shimásání = (my) Maternal Grandmoter
Shi = I, my (although you may encounter other forms, like shí (high tone) and sh- (losing the final i).
Shi nali asdzaan means paternal grandmother and shimasani means maternal grandmother.
They need to be more specific on which grandmother...