Abe's life isn't a fairy tale with a wicked stepmother. It's a superhero story. So his second mom turns out to be awesome, not evil. She's warm and loving. She's the Aunt May to Abe's Peter Parker. Yep. We went there.
Like Nancy Hanks, Sarah Bush is very supportive and encouraging to young Abe. Unlike Nancy, Sarah can't read, but she loves books, so she has Abe read to her. Hey, now there's an idea.
And then she gets him his own journal when he turns eleven, which was quite the gift: "Books were hard enough to come by on the frontier, but journals were truly a luxury—particularly for little boys in poor families. One can only imagine Abe's joy at receiving such a gift" (2.17).
Think of it this way: if Nancy Hanks started Abraham's education, Sarah Bush does a bunch to encourage him. She enables him to continue his education on his own terms. He no longer just reads the work of others—he writes his own. It's thanks to Sarah that we have this book in the first place. Where would Abraham Lincoln: Vampire Hunter be without Abe's journal?
In fact, all this works out so well, and he likes her so much that Abraham starts calling her "mother" later in his life (7.65). That warms our hearts as much as Abe staking vampires in their hearts.