I was thinking the same thing! I'm not sure why, because i've only seen 2 seconds of them together, but it doesn't seem like they have the same relationship as Trinity and his other kids. She seems less afraid of him, and I don't see Trinity ever letting his two other kids not attend thanksgiving no matter what age they get to, not with the level of control he has over them. I mean he tries to strangle his son and all the wife and daughter do is sit on the floor crying!? WTF is that?! Too much fear not enough rage they should all get to gether and bash his brains in. Ah well...so I'd say on a guess that she wasn't Sally's. But then who is her mother? IT seems like it was definitely her that shot Deb and Laudie so maybe her and Trinity bond over being crazy.

Sally and Rebecca have a different last name than Arthur and Jonah; Arthur and Jonah have the surname Mitchell while Rebecca and Sally have the last name Simmons.

But Rebecca is younger than Jonah, right?

They could be doing that new-agey thing where one child takes the dad's name and one take's the mom's name. Because thinking of other reasons makes my head hurt.

I definitely think Christine is the result of some other coupling Arthur had.