OP you've got this, you really have.
Please try to ignore a lot of the unhelpful posts on this thread. The awful situation you've been put in has caused lots of people to feel very angry on your behalf.
The most important thing is that you and your 3 year old and your unborn child get through until the birth in as peaceful a manner as possible.
Other posters have given you advise to contact your midwife. I'd second that, along with changing your birthing plans to ensure you are happy with your birthing partner, if you need one. You've got this.
Your husband, hopefully by now has been told to Fuck Off by your brother and sister in law. The situation with him can wait until after baby is born.
Don't let him have the power of being involved in your life before that. I think you have said you have access to personal money, hopefully this is the case.
His needs come last now. He's let all of you down. He will no doubt try to manipulate his way back in. Try not to let that happen.
You have done nothing wrong. Some men are just complete bastards who don't deserve what they have.
Ignore the posts about harms to your children. They will not be harmed. They have a strong mother, that's what they need right now and for all of their lives. You've got this.
I'm not sure if there will be space on this thread for you post further if and when you want to . If you find it helpful then post a follow on thread. But you don't owe any of us anything.
Stay strong .