Translation:She is the girl whose brother is away.
The definite article is required. 'van wie' can indeed be compared to 'of whom', because either should be followed by a definite article. The equivalence of 'whose' in Dutch is 'wiens' (for men) and 'wier' (for women), but these are archaic words.
Zij is het meisje van wie de broer weg is. Zij is het meisje van wie het kind weg is. (Zij is het meisje wier broer weg is.) (Hij is de man wiens broer weg is.)
This has more a technical background. The coding behind DL generally takes care of contractions such as he's etc., with some of the words this seems not to work for the full 100%.
Regarding your comment "the central committee stews over it" etc.. Most of the people active on Duolingo are volunteers, hence spending their own free time helping other people and trying to provide the best course(s) possible. With 64 skills in this course and 790000 Duolingo users that signed up for the English > Dutch course, you probably can imagine that there is a substantial amount of reports that have to be dealt with. Which comes on top of other tasks such as moderating the forums and improving the course.