What a sad, sad situation. You and your sister both went “home” to your mother to have a safe place. Instead, you are all living in chaos.
As I read your letter, I had to wonder if your mother really wanted such long term guests. It could be that she felt she couldn’t say no to the two of you. She may be overwhelmed by the addition of three more people to her home. It may be that she is saying “no” by making it impossible for you and your sister to stay there. If that is the case, it is unfortunate indeed that you all don’t have the kind of relationship where she can just say it’s too much or where all of you could sit down and figure out how to make it manageable for her.
You also didn’t mention how old your mother is and whether this is new behavior. If she is getting on in years, you may be seeing the emergence of a medical condition or dementia. Do consider taking her for an appointment with her doctor to make sure she is medically okay.
That being said: There is no reason for you and your sister to scream just because your mother is screaming. It doesn’t help the situation. In fact, it only makes it worse. It doesn’t solve the problems and it exhausts you both. It is also a terrible role model for the 10-year-old who is watching all of you and learning how she is supposed to act while with family. Not good.
Just because others invite you to be irrational doesn’t mean you have to accept their invitation. Instead, you could say something like, “I can’t hear you when you get loud. When you want to talk with me quietly to solve the problems, let me know and I’ll be glad to be part of the conversation.” Then leave. When things calm down, invite your mom and sister to have a reasonable talk about how to make the current living situation workable.
Please don’t wait for others to change for you to get the peace you need to have a good last trimester. Take charge of your own part in the family dynamics. Change your reactions and withdraw when you need to. Take yourself to another room. Take a nap. Take a walk. If the other women won’t work on making things better at least you can take yourself out of it.
I wish you well.