Correct me if I am wrong...it happens a lot...I think that the fact that she booked after knowing one important factor to you and the FI was THAT no one had been there before. There hundreds of resorts to go to, why did she have to choose this one, after you booked it for your wedding. It's not an Aunt, friend or another wedding...it's your future mother in law. Again, I don’t know the facts and if it was booked after or before..but I would be slightly annoyed.
I also think that EVERYONE has a different relationship with their FMIL or MIL and that can sway someone's opinion of this.
On the bright side, she may have some awesome advice...and like I said, keep looking for those deals, site visits are great experience if you can do it! Overall, it’s your day, you are paying for it and it will and should be the wedding of your dreams. Like some other girls have said on this forum about MOGs and MOBs, just give them a designated To Do List. =)