Grandma can also be 'isoäiti'
Yes, that's what I learned but I asked a cousin and he said mummo was more familiar. Still isoäiti should be accepted.
Maybe Duo tries to differentiate between mummo=grandma and isoäiti=grandmother, because there is a difference in tone (formal/informal) that is actually the same in both Finnish and English?
Yes, it should. It doesn't much matter if there is a difference between "grandma" and "grandmother", tomato, tomahto.
There are many things you hear people call their grandma. "Mummo", "isoäiti" and "mummi" are the most popular.
yeah, it should accept "isoäiti" but it doesn't
'Isoäiti' should be accepted
today it was accepted