1) proof she has received a lot of money from the family already
2) proof that she does not take care of her daugther. She only wants to keep the daugther on her side so that she does not have to work and your brother has to pay to support both. If your brother does not pay enough, then the daugther will be mistreated., ie. daughter is the hostage if given to her
3) proof that your brother and your family members provide a far better environment for the girl.
4) sign separation agreement first. Five year separation=automatic divorce, not one penny given. Hire your own lawyer to play the game, cause her lawyer will teach her tricks and you will be at a disadvantage if you don't have a lawyer to consult.
5) If you have joint account, take away the money. If single name, don't bother to transfer, the lawyer will ask to trace your money back for a few years. He will find it even if you give it to relatives. He can ask for past bank statments.
I don't think she really wants her daughter living with her. She cannot work if she is a FTM. She justs want a lot of money so she can live an easy life without the daugther. The more your brother shows he cares, the more money he will lose.