Step Dad in need of help

[maninpa]

We live in PA. and my wife is planning on getting a divorce. I have been with my step daughter all her life. (Before birth) My question is how old does a child need to be in order to decide if she could live with me or her real mother. She is currently 14 years of age. She does not like what her mother is doing and wants to stay with me. I never was able to adopt her. :( I really need help! What can she do to stay with me!

Thanks