I agree with BGM and Wendy. It’s not the nicest move to put your wedding two months before hers. I think you know this, but you’re bitter that MoV held off on the engagement because of SIL’s feelings so you don’t care.
If your brunch place has an availability in the fall, AND if you haven’t sent out Save the Dates AND if you won’t lose your deposit i really think you both should consider moving the date.
There’s nothing here to propose that SIL would be upset that your wedding is happening six months ahead of hers like you suggest she would be. Wendy and BGM have already stated why two months seems too close. If anyone gets upset at October you state you want to get the family making happening. 😉 Honor your grandparents by helping celebrate their anniversary on the actual date rather than wanting them to celebrate by celebrating you.