सात लोग उनके परिवार में हैं
Is duolingo being too rigid, or is this not a proper hindi sentence?
why is उनके used instead of unka?
At a guess (also a beginner), I'd say it's because when we add में as a postposition to "their family" it puts it in the oblique case
Can we use "unhe" instead of unke?
उन्हें = उनको
Which is a different use case.