Well the confusion in this question and your prior one was answered in the SWH of the other question, as follows:
"Sorry for confusion. This question was not my own. It was from my sister in law who, as you can see, is in a better situation, than most. SIL doesn't sign up for forums and I was telling her about this site and she asked if I could post a question regarding the one asked."
As to the erratic behavior of your husband's ex - she's either insecure in herself or in her marriage (which is why she can't handle the rejection of being left by your current husband and the love that her daughter (your stepdaughter) has for you, OR she has some form of mental illness or personality disorder that causes her to swing from mood to mood. She sounds like my husband's ex almost to the letter.
What I've learned is that you just have to keep your distance, let your husband handle her, say as little as possible in front of the stepdaughter (at most, when Mom is raging, say, "Oh I'm sorry she feels that way. I'm sure she'll change her mind when things settle down for her." or "Sometimes people use very strong language when they are frustrated or angry. Don't worry about it." Change the subject.
Don't keep trying to do nice things for her because it makes you look even more accomplished, you know? Be nice, be civil, but don't give anyone in your life the story you have given us here about never coming between them. I know you mean well, but it comes off as so noble that someone insecure, like his ex, can get even more threatened.
And I'd be careful about letting his daughter know she is always #1. That can actually serve to undermine his marriage to you (if push came to shove, would he choose her over you? Doesn't that empower the ex wife?). It will also change her view of her dad's relationship with your children. If she's #1 with her dad, then your kids must be #1 with you, which is contradictory to your statement that you love her the same, equal to your own kids. Make sense?
Let your husband handle all the childrearing stuff with the ex, even if it's stuff you would normally handle. Take a back seat, don't be obvious with your mastery of the blended family stuff (which may feed into the mom's insecurities), and just make your home a haven for everyone but without sacrificing your own needs as a wife.
You can't fix people sometimes. You know that annoying driver behind you tailgating? You can wish for him to be nice and calm just like you are, or you can pull over and get out of his way without confronting him or trying to sweet-talk him! Don't get walked on by this mom, but don't always be in her face with either your calm demeanor or your asking her to be different. Some people just aren't reachable. You didn't cause it, you can't fix it.