I just tried "She is my sister" for the fun of it; because I read in the comments to the Parents exercise that this was the common implied reading for family members and thought it was worth trying. It was wrong. (No problem though; it's fairer on the learners to keep it quite literal, I think.)
Because this is early in the beta release, there are a few possible explanations for this:
Since this is only the first lesson, it is as you say and they're deliberately keeping it simple for now.
It was an oversight and they forgot to add it to the database, in which case we can flag it in-lesson and report "My answer should be accepted."
I don't know how the behind-the-scenes coding works, so I don't know if the course can tell that we haven't gotten to the more advanced stuff yet and will unlock the other possible answers in the database once we've gotten to those later lessons, or if we need to flag it and report "My answer should be accepted" because the course contributors forgot to add it to this question's database.
Gender and age have nothing to do with it. The course contributors need to manually add each answer to each prompt database individually. There are bound to be inconsistencies.
In general, the preferred syntax in Latin is subject-object-verb. But "esse/to be" is a verb of state, not a verb of action, and therefore the preferred syntax there is subject-verb-complement. Since the subject and the complement are generally both in the nominative, this helps keep them separate instead of being interpreted as all just the subject.
Could "She is sister" also be correct? I'm confused because in the previous question the answer was "You are father" and there was no option to add an article before "father" whatsoever. So I figured the same would apply here, but apparently the answer has to be "She is a sister"..... Someone explain pretty pls