Is there a possiblity she did not trust your intentions (motives for the trip) or think that the trip was not what you were actually doing (possble cover-up)? Does she have issues with your sister? Just thoughts. Has she ever indicated where her vulnerabilities are with you? Could this have been a situation that was tapping into something more than you are seeing on the surface? Think hard. She has to have given you some clues if she hasn't told you straight out what the reason for her upset.