"They are my older brother and older sister."
It is conventional in Chinese to omit the possessive particle when talking about close relatives.
I think the point of Duolingo is to gain a more literal understanding of a language. Its hard to convey/teach subtle colloquiallisms like that.
One of the Chinese sentences is this lesson is formed without the possessive. It doesn't make sense to not accept it as an answer.
Yes. We use 她們 only in the case all of them are female.
BTW There is a less learned character 妳, being the female version of 你. We can see it often in Traditional Chinese ladies' media. 妳 and 妳們 are also used in the same way.