What do I think? You need to stay out of it. It isn't any of your business.
OP, I don't think you have enough information to make a call on this and, as others have said, I think you should stay out of it. You could cause real problems when nothing exists. As for deleting text messages, everyone has different habits. I delete all messages/calls I get from my phone as soon as I have read/heard them. Just because you don't, doesn't mean she is trying to hide anything.
Thanks guys. I never intended to confront anyone about this. I figured I would just let things play out like they will. I hope it is not true. I love my DSIL like a sister and would hate to see her screw things up.