Translation:I gave my sister two white sweaters.
Why the translation 'I gave sister...' (without 'my') isn't right? There is no 'моей' in sentence.
True, but the program here is inconsistent. In other exercises, if I add an article like "my" to the English translation when none was indicated in the Russian, Duolingo tells me that I'm incorrect. I understand both sides here, but I just hope that the inconsistencies here are dealt with.
This is true.
I stand by my statement above, but I've run into the same problems you have with the "assumed my" in previous lessons. It only seems to happens in lessons early in the tree. (but I could be wrong)
I suspect what the creators were going for, early on, was to just translate what's in front of you. I know when I try to learn languages I tend to insert words that just aren't there.
That said, I agree with you on the consistency/inconsistency. I'd say this could be something addressed in the tips, but reading comments on the questions I don't think a lot of people refer to those very much. If at all.
That said - overall I think this has been one of the best courses I've tried out here and I think the creators have done a fantastic job with the variety of material, the sneaking in of new stuff on the review and pretty much everything else. So I can forgive them for this, mostly :-)
From skill Numbers 2: USE OF ADJECTIVES WITH NUMBERS In Nominative expressions like "2/3/4 big cats" the agreement of the adjective may not match the noun. For masculine and neuter nouns, you use the adjective in Genitive plural.
If you're asking why this is, there's probably no answer. Languages always have exceptions :/