Why the hell should the OP hide her feelings when the PIL's choice to ignore their daughter's snub just enables her shitty behaviour. I would ALWAYS make my feelings plain about that!
The OP's choice NOT to gloss over the SIL's actions is just as valid a choice as the PIL's and her DH's choice to enable the SIL to continue behaving in a shitty manner without being called on it ignore the snub.
One does not cancel out the other. The OP is perfectly within her rights to shout out and be cross with her SIL's behaviour. If that caused a row, that is because she was making it hard for the PIL and her DH to continue ignoring the SIL's shitty behaviour.
OK, that might have been hard for the PIL and the OP's DH. But she has spent 4 fucking YEARS doing it their way. Maybe now they should spend 4 years doing it her way, confronting the behaviour and calling the SIL out on it every time she behaves like a shit.
If not, then they should not have a problem with the OP removing herself from the situation.
Why should the OP be the one who has to take all this, not saying a word, when it's mostly directed at HER. Surely then the OP gets the choice about how to react.
And if her DH starts issuing ultimatums that she HAS to accompany him to these events, and that he WON'T stand up for her to his sister, well, then he's a cunt. That's emotional abuse IMO.
If he does that, I'd be pointing to the door and telling him I hope it hit him on the way out!
And as for not listening to the voicemail, how in denial about his sister's shitty treatment of the OP, who he is meant to love, IS HE? He is so far in denial that he won't countenance listening to a voicemail that proves it to him, because it might force him to act.
I'd start playing the voicemail on fucking speakerphone and MAKE him confront just what a shit his sister is being to you. Don't tell him, just do it!
Break his ability to sweep this under the carpet, no matter what the costs to you are.
But then, I won't tolerate this shit. When my MIL was acting like your SIL, and my ex refused to confront it, I recorder her hissing to me that she wished I was dead, and that she would dance on my grave. He had no choice but to listen and confront his denial, and how much it had affected me, and how much he had been asking of me to continue to pussyfoot around her rather than remove myself from the situation entirely!