@ver, some people are seriously crazy about their weddings.
Don’t change the date of your wedding. I’m guessing the only acceptable time for you to get married is at least six+ months after your soon to be SIL. She can not dominate everyone’s lives until her “big day.”
However, you (or your fiance rather) needs to proactively keep the peace to remain on friendly terms (if that’s what he wants). I think he should calmly explain to his sister and his parents the importance of this day for you both and how it’s a simple wedding and will in no way take away from bridezilla’s wedding. There might not be any reasoning with her, but at least his parents will hopefully understand. She has A TON of weekends to pick for a shower / bachelorette. She does not need to hold these parties exactly 8 weeks out.
Don’t divulge any extra information about your wedding or honeymoon to the sister. The less she knows the better. The less she knows, the less she can find ways to get upset.
My close friend and I got married about 8 weeks apart. She was into the traditional wedding thing. Wanted all the parties. Etc. She also wanted so badly to GET married. I was meh about the whole wedding thing, I just wanted to BE married to the husband. I ended up getting married “first”. All of our friends went to both weddings (in different cities). I went to all her parties. It was fine.
One of my cousins got married three weeks after my sister. Again, in different cities. Nearly all of our family… even those who traveled… made it to both. Again, it was fine.