LMo, the only advice I can think of is to just assume that she's not going to do anything with you, so you won't be upset when she takes a "better" offer. Unfortunately, it doesn't do anything for wanting to know when and for how long you need to be feeling on guard. I guess this would be an occasion for some firm boundary setting.

Also, I'd talk to your stepbrothers and let them know that you have firm plans with your parents and to please arrange their visit times with their mom accordingly. I don't know, tell them with your new job assignment that you aren't as flexible as you used to be. It's not a complete and utter lie.