You cannot gain custody of an adult, after they have become an adult. You are also not able to collect child support if you did not start an action prior to your brother turning 18. This, of course, may vary by state.
An additional problem you have is the authority of New York over a woman who resides in Virginia. There may be no jurisdiction over her so a New York court could not order her to personally do anything, including pay child support.
Consult with an attorney in your area.