Sick to my stomach

I need advice from anyone whose been through this. We have been saving money for a lawyer for custody of my step son. Unfortunately his mother is your typical 30000% bitter baby mom. Withholds the child, manipulates him, etc. the list can go on for days with the damaging things she does. Anyways we have a lawyer and the paperwork we are just saving for the huge retainer they want. My fiance wants to go for full custody after she fails (and yes she will fail) a mental eval. And a mental eval on my step son will show how she mentally abuses and manipulates him. My fiance got papers in the mail yesterday for court that she has obtained council and wants primary physical custody and joint legal custody. Due to “she has had him herself for 4 years.” Yeah no shit because you are fucking alienating him from us. I don’t know what to do here. With Christmas around the corner we do not have the rest of the money for the retainer. But I guess my bigger fear is what leg does she feel she has to stand on ??? Why wouldn’t a judge grant my fiancé 50/50??? The ONLY reason she is doing any of this is because of me. And because of control. If I wasn’t in the picture she wouldn’t be doing this. Her hatred and jealousy for me is stronger than her love for her child. I wouldn’t be surprised if somewhere in that paperwork she requests me not around him. Although I’m his fiance. Can she do that? What should we expect and or do? What should we bring?