I want to get married during future sister-in-law's wedding
Why did you want to do that?
I think it would be fun and I want to be in a loving relationship again
It is wrong to steal someone else's wedding date. You should respect other people and not try to disrupt their ceremonies. Your extended family will look down on you if you proceed.
I could be fun if it caused a lot of trouble to my ex.
From the way you describe it, you are trying to ruin your ex-husband's sister's wedding? While you are not under any obligation to attend the wedding in that case, I still think it is bad to deliberately choose the same date for your own ceremony. This is just petty, and it really doesn't help you to accomplish anything.