Small world: the king (who? check out his "Character Analysis") just so happens to sell Jim to Tom's aunt and uncle.
Aunt Sally and Uncle Silas are good-natured and hospitable southern folk with lots of kids. The two fall for a bunch of Tom and Huck's lies, but they also have honest intentions and big hearts.
She grabbed me and hugged me tight; and then gripped me by both hands and shook and shook; and the tears come in her eyes, and run down over; and she couldn't seem to hug and shake enough, and kept saying, "You don't look as much like your mother as I reckoned you would; but law sakes, I don't care for that, I'm so glad to see you! Dear, dear, it does seem like I could eat you up! Children, it's your cousin Tom!—tell him howdy." (32.10)
And here's Silas:
That's all he said. He was the innocentest, best old soul I ever see. But it warn't surprising; because he warn't only just a farmer, he was a preacher, too, and had a little one-horse log church down back of the plantation, which he built it himself at his own expense, for a church and schoolhouse, and never charged nothing for his preaching, and it was worth it, too. There was plenty other farmer-preachers like that, and done the same way, down South. (33.26)
Don't these seem like nice folks? They're hospitable, too. When Sally sees Tom (playing "Sid") coming down the road, she says "Why, I do believe it's a stranger… put on another plate for dinner" (33). She doesn't even wait to find out who it is to start feeding him. Talk about Southern hospitality!
Unfortunately, they also embrace the South's tradition of slavery. So we're left feeling a little confused. Are they good people, corrupted by a bad society? Or are they fundamentally bad, unable to see how wrong it is to own another person?