Sarah Lincoln and Thomas, Jr.
Meet Abe's older sister and younger brother. We only hear a little about them, in part because they both die: Thomas, Jr. dies when he's about a month old; Sarah survives long enough to get married in 1826 and to die in childbirth in 1828.
So these two may not be all that important to Abe's development, but they do help teach us something about America back then: it was dangerous for babies and women in childbirth. Props to Mary Todd, then, for safely delivering four of 'em.
Abe's Stepsiblings: Elizabeth, Matilda, and John
When Sarah Bush Lincoln marries Thomas Lincoln, she brings along her children Elizabeth (13 years old), Matilda (10), and John (9). The older girls don't appear again, but since Abe is about 10, he and John become pretty good friends.
Abe calls John "the General" because John likes playing war games (2.215), which nicely balances out Abe's life—Abe's stepmom likes him reading and his stepbrother likes him running around outside. That means Abe grows both smart and strong. Bonus.
John also accompanies Abe on some trips. He convinces Abe to go to Calhoun for Abe's 22nd birthday (5.9) and then goes to New Orleans on Abe's second river trip (5.32). But Abe never invites him to come vampire hunting, so he couldn't have liked the guy that much. Or maybe he liked him so much he wanted to keep him safe.