Is it clear to MIL that you don't like her either? Do you both talk about each other behind the others' back?
I ask as, surprised you are sharing the SS stuff. I'd get this thread deleted, if I were you.
I'm wondering if the animosity has been on both sides.
You looked at a message that didn't name you. Based on this message youve gone through texts on someone's phone
I'd be raging too. But eavesdroppers never learn good about themselves, so it's said.
Tell your DH and let him deal with it.
& remember if someone doesn't like you then they don't. Likewise for you re people you dislike. They're not going to give you a pass because you are epileptic as you're still the person they dislike whatever the case.
Writing about SS situation on here, if someone recognised MIL from the thread for example, screenshotted what you've said about it and then showed it to her - wouldn't it amount to the same thing in terms of her talking (texting)about you behind your back to someone else?
Probably best from now on to minimise contact with them as much as you can too.
thinkingobably said horrid things due to exasperation about a particular situation a person has caused