Should I say sorry?

I'm going to cut a long story relatively short. So, I confided in my sister about something 2 months ago. It was sort of because I had to, in the sense that her friend told her about said thing already so I kind of had to admit it. I admitted it was true to my sister, but I asked her if she would pretend she hadn't heard anything and let me tell our parents when I was ready. Yesterday she told my mum, as far as I can gather "out of the blue", and I have a short temper so got pretty upset about it. I have been oddly crying on and off since and just feel really upset she deceived me in that way. My parents say I shouldn't have had a bit of a go at her because I shouldn't expect her to lie when they ask her questions etc. But like I said, I don't think any questions were asked, it sounds like she just randomly came out with it. Also, if our roles were reversed, and I didn't want to lie, I would immediately text my sister explaining what happened and apologising/asking if there was anything I could do to help. I know it's hard to understand without knowing what this thing is that was such a secret, but it was something quite personal to me and something I cared about and wanted to tell my parents about in my own time. It has nothing to do with my sister, she just happened to find out. My dad says I need to say sorry to my sister, as she did nothing wrong. My basic reasoning is just that I'm disappointed she brought it up, given our promise, and if she felt it was a burden and no longer wanted to keep the secret, she should have come to me and told me.

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