All ready having a bad feeling

Today is my step daughters bday and it lands on our day to have daughter. Her mom had her last year. Last night her mom was already texting my SO trying to keep her today. Her mom will keep her if she wants and not let him have her because they dont have a court ordered custody agreement yet. We have been waiting months for a mediation date and have yet to receive anything. Last year her mother had daughter for every holiday. My SO was supposed to get easter but we could only have her for half because her mom wanted her for the rest. It was his weekend to have daughter on father's day weekend and her mom kept her from him because she filed for custody that friday and said daughter didnt want to come over and she would not make her daughter come over if she didn't want to. My SO said he would pick her up and she would be fine but her mom said no and it's not his weekend unless she says it is because its her daughter and she allows him to have time with her. Then that Saturday she said she would drop her off and never did so we only got to spend half a day for Father's day with daughter when we should have had the whole weekend. We were supposed to have thanksgiving last year but her mom made plans and kept her. We got her Christmas eve and her mom picked her up early Christmas day. In 2014 it was our year to have daughter for Christmas and we only got her for about 5 hours because her mom wouldn't let us have the full day that we should have had and she had us drop daughter off at a friends house she was at. She came out to the car completely drunk off her ass stumbling across the street. Her mom had her for every holiday in 2015 we only got half of easter and Christmas eve. We also made it a point to go over and spend 4th of july with daughter last year because her mom was keeping her away from us so we did get to see her then but it wasn't our year to have her and it was very uncomfortable because of the tension between my SO and his ex. It was our turn to have daughter for new years eve and it landed on my SO's day(thursday)and his weekend. Of course her mom wouldn't let us keep her so we only had daughter for a few hours and then went back to her mom's just to get dropped off the next day to come back for our weekend with daughter(made no sense to me on why she needed to do that). Now this with her bday. Im getting sick and tired of this so called woman/mother getting away with what she wants. She really believes she is god gift to earth and everything she says is gospel and truth. Oh and of course she thinks she is the only one looking out for the best interest of her daughter and my SO is a shitty father tor being selfish. She is the worst example of a mother to her children or even just a decent human being for that matter. This just infuriates me to no end that she has gotten away with so much in 6 years. All my SO and i want is for equal time with daughter and we cant get that because of her mom. We were looking forward to seeing daughter today for her bday and now we dont even know if it is going to happen. That has just ruined my day

Edit: i dont get involved in their conversations they do that on their own. My SO lets me know what is going on because we are a team and he includes me by letting me know. He has filed for custody like i said and we havent been able to do anything for months because we have yet to recieve any mediation date. He is going after 50/50 and his ex filed for full custody of their daughter after she said she wanted to be fair with custody. My SO doesnt just want visitations with his daughter he wants to have rights to her as well and be a father to her. He doesnt just want to be the parent she visits while her mom has all rights to her.