It does sound like you have a lot of outside forces working against you and your DH.  It's hard to wrap one's head around the behavior of your husband's family - they really seem toxic and like they should be cut out of your lives.  

It annoys me that my MIL is friends on FB with BM and likes all of her posts and comments on many of them, telling her how wonderful and beautiful she is and how much she loves her.  (Yup, BM is the woman who was a serial cheater and left her son).  It annoys me that MIL gushes all over BM when she's in town to visit (mercifully she lives many states away).  BUT - MIL does all of that with me, too, so it's not like she's favoring BM over me.  And she treats all of her grandchildren wonderfully and my DD is every bit as important to her as the SDs are.  So even though I will never understand how she can continue to treat BM as the cherished former DIL rather than the cheating trollop who hurt her son, my situation is quite different from yours, where you and your children are treated as second class.  That is completely unfathomable to me.

I know it's easier said than done, but removing BM, SIL, MIL from your headspace will help clear up a lot of the chaos you are feeling.

For me, the best cure for feeling anxious/hypervigilant about step and inlaw issues was complete disengagement and TIME - a prolonged, continuous level of peace and safety, and training myself to stay out of the fray. Drama flare ups did happen, but each successful instance of DH handling them built a healthier foundation as I learned I could trust him to protect me. As he built a track record of keeping his crazy baggage away from me, I felt safer and happier. I intentionally busied myself with friends, hobbies, and new projects, too. The drama can become a bit of an addiction, so it helps to invest in positive distractions.

Have you considered sitting down with your H, explaining that issues with his family are affecting your wellbeing, and asking him to shield you from ALL of it? All changes require effort and time, but once you both get into the habit of the new norm you'll find a lot less stress and negativity in yourt life.

I feel the same way about SDand my in laws, less with BM because DH doesn't really talk to her, however my in laws act like the middleman and talk to her which is a threat to stability. It really is so nice and peaceful when my in laws and SD are not around and not talking to us. SD is either ignoring us or has us blocked. My DH was annoyed and upset with it at first but it's not bothering him anymore, he's free from careing nwo because she wants nothing to do with him 90% of the time.

I wonder if part of the pain you are experiencing is disappointment.  

I don't have a relationship with my family so when I met my ex's family, and they were warm and welcoming at first, it felt nice and I felt like they were going to adopt me, and we'd do all kinds of fun family things together.  They turned out to be aholes, and that was very disappointing.  It would have been so much more disappoiting if we'd had kids together, and his family didn't love them.