I have 2 stepdaughters age 12 and 14. I have been in thier life in this role for 2 years. I have one son Age 12.  SO and I have been friends since we were kids, we now all live together in my home.  SO and I couldn't be happier with each other. Over the last 2 years I have invested a great deal into building a relationship with his girls. I have always made it a point to treat them no differently than my own child. SO having primary custody has welcome the help and always expresses his gratitude.  Unfortunately,  as of recent BM has decided to become more involved in thier lives. Which at first SO and I were excited for the girls because they have always wanted a relationship with her. Unfortunately,  as time has passed she has not changed and her sole purpose of her new found relationship with the girls was out of jealousy that they had developed a relationship with me. She has forged to create conflict through parental alienation,  using the girls desire to have a relationship with her as leverage for thier loyalty. It started small and has grown significantly. From her berating them for texting me. "Oh, you like her so much, then she can be your mother." To now her telling the girls that the reason she hates me and they should to is because I bad mouth her all the time. Because I have never said a bad word about her to the girls because I dont want to hurt them. She even goes as far as to tell them that I do it through messages to her. Trust me when I say I keep all contact with to a minimum because she is borderline and likes to argue and stir up conflict when thier isnt any.  Unfortunately,  over the last 2 months this has caused them to turn my house upside down. They show up after a visit making it abundantly clear they do not want to be here. BM has also begun allowing these young ladies to live a lawless lifestyle at her home, purchasing them clothes that barely cover thier underwear,  allowing them to speak and behave any way they choose, profanity is allowed and encouraged as cute and funny. They speak to thier father by calling him by his first name and screaming in face if he tries to redirect thier behavior.  Because I am experienced working with troubled youth I use behavior management to address the issues instead of trying to discuss it, because it only provokes am argument which is what they want so they can use gaslighting to try and confuse the situation. When thier is an issue a just post a rule to address with specific consequences for violating that rule. This way it is being addressed to the household instead of one specific person and it sets a boundary.

Unfortunately the girls telling thier mother about my parenting style has lead her to convince them that me doing so is abusive. So now they have this anger and hatred directed at me. I cannot say anything even if it's just trying to have a conversation without the oldest jumping down my throat and telling me how I mistreat her.   

The reason I have chose to disengage is because I am human, I do have feelings for the girls. It does cause hurt and pain and I need to be there for my kid too, who gets lost in the shuffle because he is so laid back and easy going.  I think I finally decided to waive the white flag when the oldest looked me straight in the face and said to me the only one that loves her or does anything for her is her mother. My jaw dropped and this immediately after I spent $200 to take her and her friend skiing.  Her mother doesnt provide thier clothes, medical care, go to school meetings. I pay for and transport them to all thier extracurricular activities. I have taken them to concerts, broadway shows, on vacations and have included them in everything.  I have had to purchase multiple sets of clothes because thier mother doesnt buy them any, so they take what I buy and leave it at her house, knowing out of necessity they will get more.

I can only best describe my current situation as being in an emotionally abusive relationship and to maintain my own mental health I have regain control. The only way I can see it working is to step back since I am the intended target and let dad parent and provide. I will still be there as a support for him, the girls are in counseling. I feel it's best I focus on my relationship and being a parent to my son.