M.B.
Well, one thing about reading someone else's texts is that what was originally a teasing message ends up hurting someone.
She may have taken what you said in a teasing way, and was teasing back, and not really meaning ANY of what she said. She probably didn't realize that YOU had sent the message, not her SIL..... You weren't able to see the person and actually interpret her body language..... text and e-mail can be a problem in this way.... we interpret it at face value, where it may have been intended to be a light, joking message in the first place.
Unfortunately, this is all too common.
Next time, don't mess with someone else's phone.....
And yes, things like that do hurt.......