3 kids with 1 on the way! 2 skids, obsessed stalker of a BM... ugh!

I was in a miserable marriage but had two amazing kids! My son will be 10 in Dec and my daughter just turned 8yr. I left my husband and over two years later we are still battling in court. My FDH and I have a 18mon old son and I'm 5mon pregnant with our 2nd child together. He has a daughter9 and son6. A psychotic bm, who is still obsessed with him as well as me. FDH and I don't fight much, except when sd9 is here or bm stirs up drama. I have been around the skids for 2 yrs. First year things were kosher, bm was whoring it up and dumping the skids on he parents and us as much as she could. FDH pays CS every month auto from his checks. Sd9 and I got along fine, I would do things for them and with them. There were bumps along the way bc of the skids poor behavior, but it was some what manageable. End of Dec 2013, BM dumped the skids off at her mothers. BM's excuse was it was easier for skids bc she used the mother address for their school to evade zoning. This just happens to be the same time BM met a guy, got knocked up in Jan 2014, and married in April. BM moved in with new husband, his mother and his nephew in a 3bdrm duplex, in a shitty neighborhood. FDH and I have been engaged longer than BM has even known her new husband. But I'm always "Just the girlfriend" and the man she barely knows is the kids Stepfather and her husband! Bm's mother is a piece of work, drama whore and is better than EVERYONE if you let her tell it. From Jan 2014-July 2014 BM's mother had skids. Slowly poisoning Sd9 against me. Telling her she doesn't have to listen to me, she doesn't have to use her manners and to make sure to run back and tell her everything I do wrong. Bm and BM's mother teach the skids that you display how much you love the skids by how much money you spend on them! And if skids don't get their way, then all they have to do is throw a massive fit and BM & BM's mother will give them WHATEVER they want just to shut them up! Yeah... that does NOT fly in Our household!! BM got the skids back in July and that is when the problems truly escalated!! Sd is completely disrespectful, Rude, uses NO manners, ungrateful, argues with EVERY little thing, talks over you, throws hours long fits, screams and cries, lazy, demands her way, talks shit to you like she's an adult and is just a mean and nasty little monster. FDH works over night, so yay, I'm the main one to take care of them. And it usually ends up that the weekends we get the skids he has to work the whole weekend or most of it. Sd9 has blatantly told me that she will break up FDH and I at whatever cost. She didn't like my response of "Sweetheart, I've been there, Done that... But I did it faster. So good luck". I have tried and tried to make any sort of relationship work with her. But I'm DONE. She instantly makes you regret doing ANYTHING for her!! Apparently EVERYTHING is my fault, according to BM and SD. Like the 7months the skids were with BM's mother they packed on serious weight! SD9 went from a size 8 in girls (at 8yrs old) to a size 7 in juniors. Ss6 went from a size 5T to now a size 10-12 in boys. My BD8 is 1 yr younger than Sd9 and BD is in a size 6-8 in girls and there is a good 60-70lbs difference between them. BS10 wears the same size as ss6, and has about a good foot and a half on ss6 in height. But BM blames me for the skids weight gain! All of my kids are at a healthy weight and have never had weight issues. But its all me? lol ok. Recently the skids have been shitty to our 18mon old. Ss6 has pushed him, sd9 locked him in a bdrm and laughed as he cried. They both constantly take his toys and are always in his face. It's to the point where our 18mon old will push them out of his face, he runs away from them both and cries when they try to hug him. My oldest haven't had to have much interaction with the skids, thankfully! But we will have the skids and my kids for this Christmas... joy! I do not trust sd9 around my youngest. Few months ago she completely unbuckled his car seat before we left and we drove around for almost 10min before she finally opened her mouth! I LOST IT! FDH did as well. Neither skid listens for shit, and sd hasn't listened whenever she was allowed to hold our youngest as an infant, She almost dropped him while she tried to stand up after I clearly told her NO! I caught him, so he was fine.
Sd causes a massive problem every time its our weekend. Bm feels she needs to have "talks" with FDH every week! she refuses to text or send emails bc she wants him on the phone. FDH prefers written communication to protect himself as well as he doesn't want to have to spend a hour or two on the phone with BM. And every time its ALWAYS all about BM's feelings. BM speaks to Sd like she is her best friend and like she's an adult. Last time sd was here... she was being rude an disrespectful. I told her I would not tolerate being disrespected in my home. She than told me it's not my house, it's the skids and FDH's house. And her daddy pays the bills so it's definitely not my house! that was after being disrespected for a hour and causing FDH and I to fight, I was at my breaking point! That's not something a 9 yr old comes up with on her own.. her mother planted that in her head. BM is always doing crap like that. Sd's new thing is "I'm soo mean when daddy is asleep, but I'm nice when he's awake bc I don't want him leaving me" I have pretty much tried everything but whopping her ass! Not my kids... not going to. I have done everything from hiding in my room, to ignoring her, time outs, making her write as punishments etc. I'm just so fed up! I have to take Xanax when ever the skid are here... but since I'm pregnant I can't. I'm finding it harder and harder to deal with the Drama whores... the monster of a child and the obnoxious BM. I can't talk to FDH about how I'm feeling without a fight... I'm feeling so overwhelmed and lost at this point!