"Are they sisters?"
Believe me, it's better this way. Before the update, when almost everything was written in plain Hiragana without spaces, it was impossible to tell where to separate the words and the robot voice would often merge words that don't belong together.. Writing Japanese without Kanji is like using like-sounding words to phonetically spell out words like this:
Beef ore the upped ate, when olem oast eff rhyth ing wa zritten inn plane here-a Ghana wither ow-t spade-says, it wa zim paws-able toot L we're too zapper ate thaw (h)erds an there-O-bot buoys woo duff-en mer(maid) g(iraffe) were-d(oe)s that doe-n-t bee long too gather..
Now tell me that's easier to read than the original.
Hiragana may be more phonetic, but it is essentially a set of oversimplified Chinese characters that are written down and taken for their readings. Every Kana character has a Chinese character equivalent. For example if I were to write the Japanese sentence in Kana with the original characters, it would look like this:
Besides I think looking up Kanji is easier than figuring out what Kanji a set of Hiragana characters stand for.
Since the "they" in this instance is specifically women then しまい is the correct choice. (Its kanji is made up of the kanji for older sister and younger sister together 姉妹）
きょうだい is used for brothers and mixed-gender siblings just as かれら is used for 'they' masculine and mixed-gender groups.
きょうだい is made up of the kanji for older brother and younger brother together 兄弟
Slight amendment: か is the question mark, and です is an auxiliary verb "to be", so しまいですか is sufficient for asking "are they sisters?". However, without a personal pronoun this could also mean "are you (even 'we') sisters?" Therefore かのじょたち is added to specify "they".
Yes, but we are usually told that the pronoun can be skipped if it is clear from the context what it would be. TheHoloSeraph asked if 姉妹たちですか could suffice, since it is clear from that sentence that we are talking about a plural of sisters, the same which 彼女たち would indicate.
However, as I understand it, the problem is that 姉妹たちですか would be interpreted as "Are you sisters?", because the default subject of a question sentence seems to be 2nd person (you). If your question doesn't ask something about "you", then you must provide the pronoun. This is the case we have here, hence we must have a 3rd person pronoun.
At least that's what I've deduced from my first few months of trying to learn Japanese :)
This is my rule of thumb, which seems to work fine so far:
Default person for non-questions is 1st person (I or we).
Default person for questions is 2nd person (you)
3rd person needs a pronoun or other indication of who that person/persons is/are.
There's that too, but there is a fault in 姉妹たち also. 姉 means big sister and 妹 means little sister, so the word already means "sisters" (and it doesn't end at 2 if you're thinking about it --it's the whole set).
Adding the plural-marker たち sounds strange for this sentence, especially because the たち plural is not a normal one. Most of the time you can use it like one, but for example さくらさんたち doesn't mean "people who are called Sakura" but "Sakura and the people with her." So, given the plural nature of 姉妹, the word 姉妹たち may be best for saying "the sisters and their friends." Theoretically, you might use 姉妹たち to emphasise that there are two sets of sisters or more, but usually just 姉妹 will suffice and 姉妹たち sounds just weird for "sisters."
You've also got a wa before the copula which is unnecessary and makes the sentence read as incomplete. You would expect to see a description about the sisters after the topic particle. The copula also makes sisters the topic which in this question acts as the descriptive word. Like "The sisters are.........?" instead of "Are they sisters"