I had a similar situation happen to me recently. Instead of confronting the text, I left to clear my head and think things over. I didn't want to do or say something that would make it a bigger deal.
In my time of thinking, I called on a dear friend, who is an absolutely great person at heart, and tells it like it is, and believes highly in his faith.
When I expressed my feelings on the situation, he said, that text was not for you. Had you not gone into the phone you would not have seen it. Obviously this person felt they had nothing to hide or they wouldn't have allowed you to go into the phone.
My comeback was, well, what if they forgot about that text?
He said, it doesn't matter. It wasn't meant for you. If this person was that unhappy with you, they most likely would have remembered that text and erased it before handing you their phone.
I don't know if your sister was driving or not when this happened, but maybe this advice will serve you well.
Just to note, when I saw this person again, they could tell something was off. When they asked what was wrong and I tried to brush it off, they said they knew me better than that and pushed for me to open up.
I did end up telling them that I read the text and how it made me feel. It did get blown up. But the end result was, that we resolved the issue.
I personally hate sweeping things under the rug. I don't want a pile of dirt later. Sometimes that only makes other situations worse, because then you explode with a full arsenal of weapons.
Good luck