Could the red flags being waving any faster? I don't think so. Personally, I would reconsider marrying into this family.
Think about it - you've been told to put up and shut up. This family will allow your fiance's SIL to do as she damned well pleases. You will always be subordinate to her in their eyes. She will treat you like shit and they will stand by and watch. Nobody will take your side, not even your fiance. Indeed, your fiance has already started on that path with his 'just ignore her' line. Is it possible to ignore someone who screams in your face? No, of course it isn't. And if you have told him what you have written here about her behaviour, and he still holds the 'just ignore her' line - he's a total fucking waste of space. He'd rather you be treated like this than be less than obedient to his brother's wishes. He's making it perfectly clear to you where you are in the pecking order and it is at the very bottom.
You've listed a whole lot of shit this woman has done to you - how much of it has been witnessed by your fiance and/or his brother? And if the answer to that isn't 'absolutely zero' then I reiterate, reconsider marrying into this family or it won't just be your wedding at risk, it will be your entire marriage. And once you have children you'll be tied to this entire family forever.
OP, I can well imagine the shock you may be feeling, seeing these opinions, which I absolutely agree with.
But many of us have been in relationships that involve dysfunctional adults, and we can see that you will (not might) have a huge problem with your fiance's family, especially SIL.
Personally, I think your fiance should have defended you whenever SIL started trash talking you.
Take your rose tinted specs off, and have a good, hard look at your relationship. If you want advice on potential red flags, have a look at the Relationships Board. I think there's a thread still stickied at the top that has a lot of adviice on red flags.
I'm sorry, OP, but you're taking a huge risk with your happiness if you marry into this famiy.