I'm with umm, she shouldn't have gone through the messages on his phone, it's like openng someone else's mail. Granted that he was careless to leave his phone there but don't let her use that as an excuse, she has invaded his privacy and that is inexcusable - but it's HIS privacy not yours, your SIL needs to apologise to him not you for reading his messages. If she wants an apology from you for a message you sent in privacy to your son, I think she needs to look at her own behaviour first - go nosing about, you might not like what you see. If you want to apologise to keep the peace then make sure you are apologising for the correct thing, not for having a mind of your own and (heavens above) sharing your views with your kids! - and make sure she apologises too for her inexcusable behaviour in reading private texts.