The intention with that game was to pick the artikels for the singular form of the words. I am a complete beginner in German, but I thought the artikel for plurals is always 'die'. However, I wouldn't be surprised if there were a lot of exceptions to that rule. As for Geschwister, yep, that seems wrong. I will fix it!
I had "Jugendliche". "Der" can make sense for a male person. But "die" would also work, for the female person.
Also for „See“. Der See - the lake. Die See - the sea.
> I am a complete beginner in German, but I thought the artikel for plurals is always 'die'.
Correct.
> However, I wouldn't be surprised if there were a lot of exceptions to that rule.
Not to this one.