Custody Arrangements

R
My brother has a 4 1/2 year old. The biological mother gave up all rights to her when she was just a few months old and my brother has the rights. He got married & had his wife adopt his daughter. They were together for roughly 3 years. 
Then she left him because she had been having an affair. 
My brother moved out of the house they shared. Didn't take much more than the clothes on his back and he moved in with me. 
The soon to be ex wife is threatening to petition for primary custody. My brother has a full time job, has a home now, a reliable vehicle. Right now they are doing a 50/50 kind of arrangement. But my brother wants Primary Custody of his biological daughter. 
He has plans for daycare, she would have her own room etc. He has everything together. 
Mom is no longer with the guy that she left my brother for and has been out partying and doing all sorts of crazy things (according to her own sister). 
In your ladies' opinions.... Who do you truly think will get primary custody? I know there are a lot of factors, I understand that... But just at a quick glance.. 
Thanks ahead of time! It's causing me a lot of stress! 😫

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