I think as the BIL is so rude and the OP so heavily pregnant he must be a bit intimidating, particularly as he seems to have full family support to treat her like a servant.
I am wondering if the OP's DH is out at work all day and the BIL is either unemployed or works from home and the OP is also at home, maybe on pre natal maternity leave.
She also sounds quite young and eight months of living with such in laws under their rules must have been quite intimidating... so I think she deserves a bit of slack and its very encouraging that she has come on here to ask for support.
It really depends if her DH will support her or if he knows/understands how awful the BIL is... or if he is under the same orders to help out family no questions and has always complied himself, or if he just agrees that's a woman's place.
If you are worried about their anger OP I don't think its a good idea to provoke them, but start with calmly telling DH how BIL is behaving and asking if he knows how long he intends to stay. Stick to the facts. Leave out the emotive language to get your message across briefly and clearly.
You need to do some information gathering first so you can make a decision on what to do. If he's leaving at the weekend or going back to Dad - then you can just let it go until he leaves and find a way to never let him back. Doesn't the baby need his room for a nursery?
Put it to DH that the BIL's aggressive behaviour and it is aggressive is causing you a huge amount of stress which is bad for the baby. And ask him to give him some rules.
But OP.... if you are worried that raising these questions will cause an angry argument and you are afraid of how they will react, that you need to contact someone like Womens Aid for advice or some of the other advice organisations that people on here may be able to suggest.
Do you have any family support of your own? Perhaps you could visit them or have them come to visit? Do you have a job or your own income? Maybe you could stay with friends for a break whilst you decide what to do.