Jack Barts kills them, but Abe never goes around saying "I'll avenge my mother's aunt and uncle!" so these old folks couldn't have been that important to our dear president.
John Hanks is a cousin of Nancy Hanks. His main role in this book is to get the Lincolns to move to Illinois. See, John writes letters to his cousin about how great Illinois is, which is still the main reason why people move there. That, and Chicago-style pizza.
We don't hear much about Grandfather Abraham—and we hear almost nothing about Thomas's brothers Josiah and Mordecai. Awesome names, though.
Grandfather Abraham is important here because he gets killed by a vampire—not by Shawnee Indians. And since Thomas sees that vampire kill his father, he's an eyewitness to the fact that these creatures do exist. He just needs to convince his son. (Spoiler alert: he does.)