Wow, that is a really hard one. You're so right, on one side you want to be diplomatic but on the other she is being a bit of a cow isn't she. I agree with pretty bride about talking with her. Maybe ask your fmil to be with you when you do it, or see if she will talk to her about it because you certainly don't want to start out your life with the family in a fight. My best friends little sister acts like this all the time, if it's not directly about her she is rude, condesending and downright ignorant about things, but we've found that talking with her helps. It's a different situation but maybe she would listen.

Are you still getting married on the date planned?

She is handling it very wrong, but maybe if you talk to her you might find out that she really did think you were trying to take over her day and her special moment. That may be why she's acting like that. Like pretty bride said, she may feel like she's losing her brother and maybe she's starting to feel like she rushed into her engagement and is taking it out on you... There could be a ton of reasons and I could go on forever but I would suggest talking to her, but not alone, make sure there is a witness there!

Good luck!