I realise it´s easy for me to say, cos I´m not living it but I think you should go NC. There´s plenty of people on here and Stately Homes etc that have demonstrated it´s entirely doable, practical and necessary.
Your OH should be respecting your feelings and really listening to your concerns. He should be supporting you and most importantly, his loyalties should lie with you.
You´re about to have another baby for God´s sake! You´re extra vulnerable. Your health ( mental and physical ) should be of paramount importance to him and everyone else. No way should you be being undermined with your feelings not taken into account.
Their your kids FFS, and what you say goes. If they don´t respect you as the mother then fuck them! It´s apalling how they are treating you. I know sometimes it can be subtle things so you question yourself and literally can´t trust your own judgement so you think you´re going mad. 
It´s your life and they´re your kids, draw a line, stop accepting piss poor behaviour and most importantly, stick up for yourself and stand your ground. Explain exactly, to their faces why you did/said what you did etc so everything´s out in the open, no misunderstandings and they can accept it, twist it whatever.....you are NOT being unreasonable to demand respect and to be listened to.
If you say to not do something re the kids and they do it, fucking well give it to them both barrels. Do not be a victim in all this. And if your OH sides with them than you´ve got some stern talking with him and consequences have to be applied. If he loved you he would empathise and support you. By disregarding your feelings and concerns he´s basically saying he doesn´t care or give a toss about you and your standing within the family.
You married him not his family. If they can´t accept and respect you then Fuck Them! You´ve given them ample opportunity up until now. Make your feelings 100% known to them, if they fail to respect your wishes as the mother of the kids, and also as an individual then tell them you´ll have nowt more to do with them.
You cannot allow them to get away with them continually undermining you and get away with it. It´s insulting and your OH should recognise that instead of being a Jackass. Be 100% honest with your feelings to everyone, they can like it or lump it bt at least your conscience will be clear. You tried, they´re still wankers, wash your hands of them.