I see nothing wrong with being truthful with her. She needs to hear it. Of course, you wouldn't say it harshly. You would say something like, "Sis, Dad was right to be concerned about what you did. I wouldn't want you to do something like that to me, and I wouldn't do that to you. You should have more respect for people's privacy. Of course I agree that Dad was right to get upset"
This is something I've been thinking a lot about lately in many aspects of life -- idly sitting by while someone who is in the wrong complains about how their bad behavior wasn't well-received. When we either say nothing, or actually agree with them when we know they were wrong, we are in part allowing their rudeness to continue unchecked.
Of course, we can't make adults change their behavior, but we can at least let them know we don't agree with it. I *know* how hard it is to do that when you are naturally not a confrontational person -- I've rarely ever done it. But it's a change I'm trying to make in my life.
For instance, the first thing I thought of when reading this post was a guy I dated several years ago. He started to display a lot of bad qualities, so I was thinking about when and how I should break up with him.
One day, he told me his cousin had called him and was upset with him. They were not close, but he had recently been to a party she'd thrown, thought it was awful, and told her so.
He then bought and mailed her a book on how to throw good parties with a rude note along the lines of, "Here, maybe this will help you next time." She was insulted, and he claimed he couldn't understand why!
I was in shock when he told me this, and fully understood why she was upset, but I couldn't bring myself to say it. I couldn't say anything! All I could do was look at him in silence thinking of what an obnoxious jerk he was as he went on about how grateful she should be to him for the book.
But to him, I'm sure my silence was seen as agreeing with him that his cousin had no right to be upset. He had such a pleased smirk on his face. I still regret not telling him how I felt in that moment. There are several reasons why I felt I couldn't reply safely and truthfully in that particular environment, but it doesn't change the fact that I wish I had.