In chapter eighteen of The Adventures of Huckleberry Finn, by Mark Twain, William Shakespeare's famous romantic tragedy, Romeo and Juliet, is hinted at. Here's why: Sophia and Harney belong to a pair of fueling families: the Shepherdsons and the Grangerfords. The young lovers decide to run away and elope, and just like Romeo and Juliet use other people to deliver messages back and forth, Sophia and Harney use Huck to pass a note to each other. The plan is to meet up at a certain time so that they can run away and be together, and eventually marry. Their plan, however, has tragic consequences: Buck Grangerford and another relative are killed in a shoot-out with the Shepherdsons. Huck feels like he is to blame since he is the one who knew what was going on and delivered the note.
When these two lovers elope, the famous bit of literature that Twain is alluding to is the story of Romeo and Juliet, made most famous by William Shakespeare. The couple you mention come from opposing families in a feud. In this way, they are like the main characters in Shakespeare's play.
In Romeo and Juliet the two lovers are from families who have hated each other for generations. They have to meet secretly as they fall in love. This is very similar to what is happening with Sophia and Harney.
The two stories share some other minor similarities, but this is the main way that this episode alludes to Romeo and Juliet.