Question for those who are good at staying calm. My SIL is really quite awful. She upsets my DP, my MIL and my other SIL quite frequently and never shows any remorse for doing so. My head is saying stay calm, be polite, it is not your battle. My heart is screaming arrrgh and wanting to say what I think. It's a new situation for me (well, 5 years with DP but it has only been bad for say last two) as my family is fairly lovely and has no-one toxic like this.
I need some strategies, I need to know what to think to stop me losing my temper at her brattish behaviour. Please advise if you can.