Thanks everyone for their replies so far.
As for removing just about everything from their rooms, we've considered it but other than aggravate them and entice another power struggle between us and them, would it actually help anything? I'm not trying to discredit the idea, I promise, I just to want to create yet another battle. The things we have taken away, phones (they had them when they moved in and were removed from our home in just a few months after it was apparent they were causing much more harm than good) their kindles, PlayStation and allowance and tv, they say and act as though they don't care what we do with them. This hasn't stopped us from still taking these things away but getting them back has definitely not been proving as enough incentive to correct their behavior.
As for their mother, I don't believe she's been dating much at all. She moved herself and the children out of state before their divorce was even finalized back in 2012/13. She said it was because she wanted to be closer to family. She's just a very angry and belittling person. From what I've been told by the entire family, she's always resorted to screaming and cursing and just completely tearing people down when they don't do what she wants them to do. She's very controlling and apparently has been this way to all her former in laws, my husband and the children. There's still a daughter that lives with her who up until recently, outside of being a normal teen with straight A's who is now showing much of the same behavior as the the twins that are with us. My husband's stance with all the children is if they want to be with their mother, of course that was fine and he wanted to support them but if they ever wanted to be here instead, our door was always open. Which is how we are now in our current situation. The children are very angry and very guarded. They don't open up and are always on the defense.
With our toddler, they've always been 110% caring and nurturing to him. They go out of their way to help the baby or to spend time with them. It's as though they hate us specifically. Even when they are mad and enraged, once the toddler's bed time comes around for example, that is the only time they will stop some of their behavior and quiet down, just so they don't wake him. It's so confusing. We've discussed among each other if it could partially be the stress of a new baby coming but they knew we had been wanting one more before they moved here. And even when we found out it was another boy, they asked if we could try again later on for a girl because they think having a little sister would be fun too. I don't say this all to say that I'm not worried, because I am so much. We have cameras, no alarms though. I don't even know how to begin to juggle a new born and a toddler in the same bedroom, especially at night. We do have pets and it's the same thing, they can hate our guts and tell us anything they can think of to hurt us and yell and curse and break things but when they calm down a little, they'll go find the cat or something and hug it and take him into their room with them. So it's just so confusing. And again, this behavior only presents when they did something that would cause discipline to be handed down. So yesterday the principal called, one of them acted out in school, got after after school detention, the child was grounded for the rest of the week and could possibly earn their privileges back by the weekend with good behavior. He listened to what I had to say, said OK, then shortly after walked out the front door. I'm not going to try and stop him physically, I'm nearly 8 months pregnant. But if it's a normal day, no issues at school or with each other, they will come in, tell me about their day and their homework, do their chores, it's "yes ma'm, no ma'm", they want to do "family things" together after their dad gets home and at the end of the night at bed time they go off, say goodnight and I love you. I'm just at a loss. We can't not ever have consequences for poor actions just to avoid conflict at home but these outbursts went from being unheard of here at in our home, to being rare now it's napping several times a month and I don't know what to do. My husband and I are both on the same page and at the end of our rope. Their school has a counselor and we've signed the consent forms for her to begin speaking with the kids but it's more of a casual, gain their trust over time thing and they don't know that any of it was initiated by us, which I feel is good, but at the same time, I feel like we're in crisis and we need drastic help. I just don't know how to get it. My husband and I are currently working with our church to seek some counseling for the two of us. We are new to the area and to the church so it's kind of been slow going but we know we definitely need it.
Thank you for all the advice and support. It means more than I can say.