Answer: Her sisters' birthdays are on the same day but in different years.
Explanation: This one may seem tricky, but there is a clear, defined answer! If there was one singular sister, we would want the apostrophe before the second "S" in sisters (because there is only one possessive article), but because the word "and" after birthday, we know there are multiple sisters!
So, we want the apostrophe AFTER the second "S" to indicate that the birthdays in question are being possessed by not one but two sisters!