I don't think there's any way to write such a letter that wouldn't be interpreted as your resentment about being left out of the will. I totally believe you--I just had to go through something similar with my folks (in their case giving equal lifetime "gifts" to family who I suspect didn't "need" it, for the sake of treating everyone the same--only one really needed the help, which would have been fine with all of us), but ultimately they did what they wanted (as was their right).
I think it's legitimate for you to express your concerns, but do you really think it will cause her to rethink it? If not, I'd just let it go.