(Your message struck a cord with me, and I debated about responding even after I had typed the message below, but I think you may need to hear this.. As a step-daughter & now a step-mother, I think I may have a viewpoint that will help.)
I think your "a little about me" says it all. You don't mention your step-daughter at all.
I don't mean to be harsh, but maybe your MIL is compensating for what she sees as YOUR favoritism to your natural-born daughter over your step-daughter.
I agree she should have listened to you, but from the tone of your message, it sounds like this has been going on for a while & she might just have figured it was one more instance of your step-daughter not getting the attention she deserves.
And before you get angry, I should tell you that I have been in BOTH places - I have BEEN the step-daughter and I am NOW the step-mother to an 11 year old boy. I think it may be easier w/ a boy b/c there isn't that natural competition for the dad's attention that happens w/ any mother-daughter relationship, whether there is the word "step" in there or not.
However, it is still difficult for ME not to favor my natural-born son over m step-son - and I say that w/ 100% honesty & humility. I don't like that I feel that way. I just do. I have to watch myself constantly & I even over-compensate at times for what I feel is a greater love for my "real" son.
I have to always be conscious that it is NOT my step-son's fault that he is from a broken home - I am positive that is not the way he would like it to be. And since I am, in a way, grateful that he IS from a broken home (because I wouldn't be married to his dad if he wasn't & I wouldn't have my son), I try not to let my resentment that I don't have the picture-perfect family impact him. Now, his MOTHER is another story... ;)
Also, keep in mind that to your husband & your MIL, your step-daughter is just as related to them as YOUR daughter is. There isn't any reason for favoritism.
I think you may want to take a step back & realistically examine your own actions & you may see that your MIL is not 100% in the wrong.
Also, therapy helps - you can get everything out w/ someone who is unbiased & will help you through it happier on the other side!