BM Not acknowledging my existence unless it suits her
I am living with my boyfriend and his 6 year old son that he has around 55-60% of the time.
He and his wife, the BM, are about to be divorced (the papers have been filed, it's a matter of weeks now), we plan on getting married as soon as he is divorced, because we want to and are madly in love, but also because I am not a resident, I have a special status as a grad student that does not allow me to perform my research or to work as much as my colleagues.
We have a beautiful happy home, he, my SS and I. My SS and I completely fell in love, we have a crazy connexion. This stable and happy environment has been very beneficial for his transition in his parent's separation.
The dark cloud of this happy life is the BM, who is not dealing well with my existence event though she is the one who cheated multiple times and wanted to split up. She feels I have stolen her husband, her house and her son. I have two main worries:
-She is completely disengaged from her son, granted she never wanted children and keeps resenting my BF for getting her pregnant, she tries to get us to have my SS more often. He does not want to go to her house anymore, says she does not pay enough attention to him, he hates the food (she imposes her vegan diet on him) and her roommates are big partiers. He told us that his mum felt more like a lady he had to stay at three nights a week and his real family was us. My SS's sensitivity and clarity regarding his mother's disinterest in him is good start for suing for sole custody, but in the meantime he is very distressed before she picks him up. He acts out at school the days she picks him up, he calls his dad constantly and when he is with his mum he does nothing but sleep when he never naps with her (avoidance). Let's face it, she is not a good mother and it is harming this little boy I love more than anything and we want to help him, protect him, make him feel better by the time we file for full custody.
-The BM does not acknowledge my existence, when she drops my SS off she sits in my living room and does not say hello or look at me. She is very bitchy and sometimes tries to manipulates my BF in telling him he is an awful person for forcing me into motherhood. None of this would matter to me if I weren't constantly doing her favors. I go out of my way to babysit my SS, she is always late or way too early. She refuses to communicate with me and goes through my BF who is working and can't just be the liaison. She is rude, toxic, she tries to break my couple and she is enable to co(parent with me even though she is more than glad to drop her son off to me.
What should I do to be the best SM possible and act in my SS's best interest.