I'd say a good start is to read the book, "Stepmonster". I know I'd like my SO to read it, so that when I say I've got the hardest part of this relationship, he's got a better understand why it is I say that.

since you admit the crap that your daughter and ex do to your wife, your number 1 duty is to protect your wife. your wife shouldn't HAVE to tell you how to handle things. you know what you need to do in order to protect her and your marriage. first, you need to figure out WHY you resist her and her suggestions? she's only telling you the things she needs in order to feel loved, cared for and protected, something you promised on your wedding day. I don't know HOW you know that the ex talks about your wife unless you're giving her your ear. why in the world, if you know for a fact that she does this to your wife, would you even THINK about going to your ex's house for a bday party? and now, your wife has gone and is feeling good about your ex. what, is your ex setting your wife up to sink her fangs in? did guests ever show up for the "party"? or was it a ruse to get the happy little family together? and unfortunately, your wife showed up. if they feel this way about her, why did they pretend to be thrilled? you seem to be sympathetic to your wife on this site, but you aren't putting it into play physically. if your ex and daughter talk bad about your wife, stop them mid-sentence. tell them to not utter one bad word about her to you, that you will hang up/leave/take them home, whatever, but you will not tolerate it. lots of questions here.