Update:
Got up to mom and dad's in time for lunch yesterday, and we all sat around and hung out for a while before I went to the old house with mom to look at everything. That went well.
Over the course of the rest of the day, however, it became apparent that mom and dad are working no recovery whatsoever, and neither is sis. She's staying on her methadone and doing well with it, but she's not going to meetings or anything. That's fine. Her recovery is none of my business.
And mom and dad's recoveries are none of my business either.
I will admit there were a couple of hairy spots when mom would say something to me that I wanted to respond to, but I knew it would do no good, as she made it clear that she still thinks that keeping sis under her thumb is the best way to go. But I managed to hold my tongue and remind myself that there's nothing I can do.
All in all, I think this was good for me, because I proved to myself that I do have the tools I need to be at peace with them, even if I don't agree with what they are doing.