At what point should a step parents say enough from being the receiver of the hurt, pain and or anger from stepchildren?

I’m no martyr but I have been sensitive for the past 10 years of my stepchildren’s hurt they feel from both their mom and dad divorce. However, now that the kids are teens or 20’s they still tend to use me as the one they subject all the anger toward when they get angry. I know being a stepmom is maybe the worst positions to be in a blended family as we take on motherly duties, give our love and yet are given nothing in return and I have lived with this. But When do we say enough? We just want the kids as they get older to treat us fairly and if mom has plans for x-mas or thanksgiving one year that is fine and we only expect we be 1st the following year.
I came into the picture after Bio-mom left the dad for her girlfriend/lover which is named in her divorce as her partner whom she took on her name and bought a home with. She has since made the children confused by once expressing her love for this woman and taking kids to therapy so they could understand a lesbian relationship then later stating she never was in a lesbian relationship and is now married to a man. Bio-mom has visitation rights till kids are out of High School and my husband has full custody and major decision making. I think this full custody is why she has been confrontational to both him and me on many occasions throughout the years and seems to have no fear to say or do whatever she feels. Flipping us the bird in front of attorney, putting her finger in my face calling me ever name you can think of in front of the kids, slamming her hand on the back of my car, yelling at my husband in front of the kids at airport and even had the nerve to call my husband’s mother to complain and most recently asked if she would make her a cake for her wedding as she left her girlfriend and is now married to a male cop.
We are recently are having issues with our stepson 14 whom is lying and showing anger because his mom wants him to have her cell in our house without us taking it away for bad grades or looking at it at all. He is also angry because she tells him she won’t see him for 3 weeks in summer because his dad set up his vacation to keep her from him which wasn’t true and forgot to mention he won’t see his dad the same amount of time due to her vacation time. He’s starting to call his dad by 1st name at his mother’s house according to the other kids too. We don’t talk badly about her but explain the truth in dates with him and explain she can use the home phone if she wants to call him any time. This is much more of an issue as the mother isn't to speak of this childs bio-dad (per court ruling) whom she created during an affair but we have found she has ignored this ruling. We don't want to push the child further away so we do not exasperate this knowledge as we understand he is in a crucial growing point so we try to just show him how much we love and care for him despite the negative expressions he may receive while he is spending time with his mother.
Recently I spent two days looking for hiking and camping gear for one of the older step kids, 18, whom put off and didn’t organize the gear himself leaving it to me and my husband while he went off for the weekend to spend with his mother. I was taking another child of ours on the trip and was dropping off both kids equipment. After he arrived he angrily yelled out loud for all his friends, parents and me to hear (and they all looked over at me) that he isn’t allowed to take camping gear to his mother’s as we fear they will steal it. He knows he’s tried to sneak things to her house before and has forgotten gear at her house finding it months later after us searching for it. And he knows that his mother has asked the kids to take things from our house for her. Not to mention that we pay 100% for all the camping/hiking gear the kids use and she has lots of money she could put toward equipment for them at her home.
I made a trip with my husband for our anniversary and plans around one of the older kids then he cancelled on us after the fact to be with bio-mom which left us feeling really hurt and used. We also paid 100% for a religious mission out of the country for this child. The mother refused to pay any money toward it as she didn’t agree with the religious part and thought it was a waste of time. The kid passed over us when we asked to see visit him in the country and see him home because his mother insisted to meet and fly home with him. She had already created a negative experience when the kid was leaving on the mission by creating a scene in the airport. The kid already spent a week with her to say goodbye and asked her to allow the last 2 days and the airport for dad but she showed up anyway yelling. We were surprised that all the religion after 2 years didn’t stop him from lying and making up many stores to us that bio-mom wasn’t going to get him on his mission.
The bio-mom has an extreme hold on them so much so that they seem to not want to mess with her. But they are willing to hurt and take us for granted in order to not upset her. So at what point or when do you say enough? And how do you stop them from continually putting the mother’s needs, wishes and desires over ours without alienating them from us?