I'm pretty sure that will go right over her head.
Excuse me, but your SIL sounds like a complete numbskull.
I'd just start deleting her emails without even opening. If she asks about what you thought about something, I'd tell her the truth.
"I don't open your emails anymore because they have attachment I don't want to read. In fact, I delete your emails without even reading your message."
What have you got to lose? If she gets insulted, she'll either tell you her feelings are hurt, and won't send them, or get your message and won't send them.
She obviously has no consideration of Your feelings, so if she plays that card, I'd let her know that also. No arguing, just state the facts and shut your mouth.
My mother had a habit (she had many of them
) of informing you, when she didn't want to talk about something (which was 99% of available subjects) that "you're upsetting me" It wouldn't even have to be anything personal, just the world at large "upset her"
Once, after I had dared bring up one of the 5,893,262 things in life that upset her, she informed me that she was so upset she "just couldn't talk to me any more" and hung up.
Within 5 minutes, she called me back so that she could again tell me how upset she was and that "I won't be able to sleep tonight, you've got me so upset and tense I'll be up all night"
I said "So, why are you calling and telling me? Is it because you hope I'll get so upset I won't be able to sleep either? I don't upset you, you upset yourself. I can't help it if you can't sleep, and it's not going to bother my sleep at all."
That was the last time she ever said anything I said upset her. She'd lost that power.
You've given your SIL the power, she sends an email, and you do her bidding by opening it. It's your computer and email account, you can open something or delete it, it's your decision.
Once she realizes she no longer dictates your actions she'll stop.