I don't see the problem with an open marriage, really. I have friends who are married (three sets) and the healthiest marriage I witness is the one that is open. Because being in an open marriage requires trust, love, and understanding. Where as a lot of my other newly married friends are adjusting to a life where it is monogamous.

I think it is perfectly fine to be upset by the nature of her reaction. You didn't intend to do any harm at all, so of course getting that reaction from your sister would be a big shock and truly upsetting.

That being said; open marriages are more common than one thinks. If I am being honest and frank, and I do apologize, I think you're internalizing her reaction to you which was truly cold and horrid of her, and you're placing that anger and resentment against her own personal choices to rationalize the upset and hurt. Which I think a lot of us would do in this situation.

You can still be best friends. She is still your sister. And take it from someone who lost their sibling when they were only 22, don't let something like this ruin something that can be so strong and important.