I think it's because the verb is negated, not the noun. It isn't the absence of juice, but the absence of drinking.
I had the same question in my mind, but i could solve it. Direct object is accusative case, and for inanimate masculine nouns, accusative is the same as nominative. And it is not genotive because it is not after нет, and the negative is for the verb, not for the noun also :)
So does this, like in English, mean both 'My sister is not drinking juice' and 'My sister doesn't drink juice (ever)'?
Yep. Russian present tense doesn't really distinguish between present simple and present continuous.
Where is the genitive in this sentence? I assume I'm missing it as this course is on genitives but the nouns all look like nominative forms to me.
It is correct. Russian doesn't differentiate between "she drinks" and "she is drinking". You should report it.
I have learned 3 versions of drink so far. Пьют, пьешь, пьет. And I still cannot figure out the when to use them. Please help me
They are the different conjugations of the verb пить (to drink). The verb changes depending on the person:
Я пью (I drink) Ты пьёшь (you drink) Он/Она/Оно пьёт (he/she/it drinks) Мы пьём (we drink) Вы пьёте (you all drink) Они пьют (they drink)
The "ё" is usually written as "е" in everyday life. I hope this helps!
Strange that "My sister isn't drinking juice. is marked as incorrect.