Step daughter's mother undermines our efforts.?

This is one of MANY times that we have tried to instill good intentions with SD. When we have a time that we have to discipline SD, mom hones in for the rescue. She has even taken SD shopping for toys for her to play with while she is suppose to be in time out at our home. This time, we wanted SD (who is 12) to initiate her own sales with girl scout cookies. My Husband had to call SD a few times to light the fire under her *** to get her motivated and start her calls/emails to do so. She wouldn't do it. SD didn't like that call from her dad and told her mom, who did the work for her. SD often runs to her mom on issues like these. We have other children in the household and we are finding that her behavior is disruptive to the upbringing of the others. How do we undermine the undermining? Do we punish SD for manipulating the situation with her mother? She is an absolute spoiled brat and we are trying to ground this behavior.