An interesting example here. The rule of course is that an apostrophe on both nouns, as in the above example, indicates two separate possessions; in this instance, the two apostrophe s’s indicate that Brother and Sister will each have separate turns. And they do, sort of. They both get called in together (which would be ‘Brother and Sister’s turn’) but they get treated separately, though alternately. So the example above is not incorrect but provides interesting interpretive possibilities.