Translation:They are sisters.
Is it possible to slow down the speed of the audio? As a beginner I find it really hard to hear how everything is pronounced since the lady speaks so fast
That's how i was at first. But I'm starting to pick it up faster. Just keep going.
No but I would say prefer listen to it multiple times instead of it being too slow(and thus unrealistic)
If "they" refers to siblings with a mixed gender, which pronoun can be used? Does it work as in Spanish for example where mixed gender is treated as male?
かれたち is less common than かれら.
But actually if I were to say "them," normally I would use the person's name + たち. e.g. 田中さんたち
Wrote "Those girls are sisters" and got an error. Is 'kanojoutachi' only used for female-they?
Yes only females, however it still means 'they' and not 'those girls'. It's a bit tricky but since there's no gender neutral 'they', it's either kanojou-tachi, or kare-tachi based on the group you're taking about. If it's a mixed group, use kare-tachi.
I can't understand why かのじょうたちはきょうだいです isn't right!? The word きょうだい could be used as for brothers and for sisters as well. Both answers should be right. It's like a parent. Why could not i say about my mother or father... he or she is my parent. There is no restrictions by gendre.
There is a word for "brothers", which (thanks to patriarchy) is used as a standard "siblings" for all groupings that include at least one male, and then there's a separate word for "sisters". Since this sentence is only about かのじょたち (mind the short '~jo') only the latter is correct.
Isn't it correct to translate as "the girls are" instead of "they are"?