OK, her own kids don't listen to her, she is a somewhat ineffective parent? Yet, she has full custody and could take them to another state if she so pleased, at any time?
You're a better mom, both to your own kids and to hers? So, your h being going more than he is home (by a long shot! one week home, 5-9 weeks away!), he wants his kids with her to have a more effectual mom (step-mom) than his ex?
He comes home and has to have sex with her a few times in that one week, and that keeps her feeling wanted enough that she will stay with you, in your house, and you get more siblings for your own kids. This helps heal a childhood wound of yours...
But she is jealous of his time spent with you, and she is jealous of your time spent with him. And he doesn't trust her, or love her (much, if at all), but will fuck her just to keep her there so neither woman has to be a single mom while he's away 5-9 weeks at a time.
Very unusual situation. Almost Biblical.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37