If it is implied in the Norwegian, why not in the English as well? Imo, it should be 'farmoren min' (or whatever is the correct form, I'm not sure), if you want the answer to be 'my grandmother'. Otherwise it's just muddying the water; I for one felt cheated when it said I got the 'wrong' answer.
We do accept "grandmother" without the possessive, as well as "farmoren min" with the possessive for the reverse sentence.
Having said that, languages are different; in Norwegian, the most natural way of expressing it would be without the possessive, and in English it's the other way around. This is reflected in our default translations.
Weird. It didn't accept it for me for some reason. To clarify, I didn't try to use 'the' (like the original poster); I answered 'Grandmother is at our house', and it said that it was wrong and I should include 'My' in my answer. I'm glad to know that it should be accepted, though.