Can my dual citizen stepmother sponsor me for a GC?

Here's the situation: My Canadian born stepmother has dual citizenship (Canada/US), although she hasn't lived in the USA for at least 20 years.

After my mother passed away, my father married my stepmother when I was 20 years old - I am now 38. I have never lived with her (I love her dearly though!)

Is it possible to leverage this relationship into a Green Card?

Nope, step-parent is not a relationship that they allow for sponsorship if the marriage took place after the children's 16th birthday.

Here's the [url=http://www.uscis.gov/files/form/I-130instr.pdf]I-130 sponsorship petition[/ur] where it say who is and who is not eligible to sponsor.