Considering that the original text quoted is "The tracking of online activity is not just a privacy issue but an economic and political minefield," there is no need to introduce a capital letter in <em>but </em>because it is not the beginning of a new sentence.
In that version of the quote, this original sentence was interrupted by a comment that is properly puctuated.
Because ‘Sarah and her sisters has gone’ doesn’t sound right, ‘Sarah and her sisters has been’ also sounds wrong and ‘Sarah and her sisters have been’ doesn’t really make sense
