How do you deal with the ex who undermines everything you try to do to create rules, bounderies, and structure with your children?

Everything she does is to suit herself. Everything she does lately is to slowly alienate dad and me (step-mom) from children.

2 examples-SD (15) had cell taken away, all agreed, no cell til 18 n buy own, now Ex says since it convenient for her, w/ activities pickups, giving SD cell back, and nada we can do about it bc what she says goes.
SD (9) poor grades, cheated on test. On our time we refused to allow her to go to girlscouts for punishment. Ex angry as she needed to p/up cookies. 2 days later at next visit p/up. Ex states SD grounded from everything for poor grades, cheating on test. (no duh)

Of course way more to looonnnggg story. This is just my 1st post here, and my current rant of the moment.