I just got engaged two months ago and have started planning my wedding. Every detail that I mention to my future MIL she says oh well my daughter did this. Her daughter got married last May. I know that she may think she is being helpful but everything her daughter did I didn't care for one bit, even the food was blah. Her daughter and I are total opposites too, even down to our hair color and what hand we write with. I just don't know how to put it to her nicely to say that yes what your daughter did was great for her but it's not me at all. I don't want to put her off in any way or cause problems between my fiance and me. I just need some help I don't want family drama before I'm even officially in the family.
I would say, 'Yes, and it was beautiful, but since her wedding was so recent, FH and I want to do something different, so that the two weddings stick out in family members's memories.'
Another idea: How well do you know your FSIL? If you know her well enough to chat with her on this and she's cool enough not to stab you in the back, explain the situation (she knows you're different) and perhaps she can say something to her mother. I doubt she wants your wedding to be identical to hers, either.
If neither of the above will work, just don't discuss details with her. Tell her you want to surprise everyone.
This doesn't sound like a fundamental problem between you and your FMIL. It sounds like she's trying to offer good advice, and she probably just doesn't have a clue that you both have different taste. Once someone mentions that to her, the lightbulb will probably click on.
One piece of advice, though: Even if you hate every suggestion that she makes, take her advice on something (if she continues to be this involved in your planning). Pick something that doesn't mean much to you and take her advice on it. It will mean the world to her and it's no skin off your back if some detail you don't particularly care about isn't exactly right.
Vice President and Guardian of the Toilet Brush of POOP: People Offended by Offended People