Here is the site you need for births/adoptions that took place in New York. It has a free registry and downloadable forms. You can fill out the "biological sibling" form, and hopefully your mother will fill out the biological parent form. Since your sister is 18, she can register and find you this way.
Adoption Information Registry - New York State Department of Health
You will also find much information and help at adoption.com
One of the searchers here is very good at searching birthdays, so please check back here frequently, and keep your email info current in your profile. You never know... your sister may come here in search of her birth family and find your thread.
Good luck