I'm so sorry. I always find it interesting that people will accept cheating (and will cheat themselves) more readily than they'll accept a loving, respectful relationship structure that is different from their own.
My next thought is that anyone who snatches away someone else's phone and reads their text messages (which, to me, are as private as my email or my phone calls) doesn't really have the right to demand an explanation for anything they read, much less pass judgment on it.
But all of that aside, this is your sister and I know you want her to be understanding and accepting and when she isn't, it has to be hard. I think if I were in your place I might be inclined to NOT talk about it further. If you've explained the situation, made it clear that your daughter is a priority, and that no one was "coerced" ... after that, if she can't accept it, then it's her problem. I wouldn't spend time and emotional energy trying to convert her further. Maybe instead give her some suggestion of resources - either online, or books she could read - and tell her that you'd rather not talk about it further until you've both had time to think and get some distance.
That's what I think I'd be inclined to do - of course others might have some better or different thoughts.
Whatever you decide, I'm so so sorry and I'll offer virtual hugs and support, for what they're worth.