And that's not necessarily wrong, but it is quite awkward in this specific context. That's something a native speaker would say but never write, similar to, say, "there're" or "there are." Say it confidently but don't submit it to a publisher. In formal speech and writing, contracting "you have" to "you've" when indicating possession is acceptable. For example: "You've a lovely wife." "I've a great job." It's very, very formal speech, however.
"сёстры" is the nominative plural form of "сестра", whereas "сестры" (with the stress on the last syllable) is the genitive form. However, the dots over the "e" are often omitted in writing, in which case you have to rely on context to determine if it's plural or not (the pronunciation, of course, is different).
So, I didn’t realize this was supposed to be a question and my answer was: You have sisters. When I put into Google translate that very same English sentence (You have sisters), it came up with: у тебя есть сестры The only difference between the 2 is that sisters is spelled as сестры as opposed to сёстры in the duolingo sentence. So, my question is, is that change of the vowel the only indication that this is supposed to be posed as a question?
No, the vowel is not supposed to change in questions. It's always "сёстры". However there's a tendency among native speakers to not bother with those dots, so in a lot of places (including Google Translate) the "ё" is simply not used. It's still pronounced as "ё" though. Native speakers just know that it's there.
The only difference between a statement and a question is the question mark.