E.A.
This difficult problem can have a very easy fix, since you are trying to handle it in advance. When you are in labor, your health and needs should be Priority Number One! Shame on the hospital staff for failing to recognize your situation.
My suggestion would be to draft a "Birth Plan." This can contain any information about your requirements during delivery. For instance, do you want music playing? (For me, who cared, just get the child out!). Anyway, the idea is that the Birth Plan contains all of your wishes and directions.
In this Birth Plan, there should be explicit instructions about who should be present during labor. Once you have it ready, give a copy to your OB/GYN office. It will be considered part of your medical records. Also, discuss the situation with them so that they understand.
A copy of the Birth Plan should be taken with you to the Hospital. Again, you can consider this a confidential medical record and your MIL, etc. have no business reading it.
Good Luck! Remember, you are in charge! It's the Nursing Staff's job to make sure that you get what you need (and rid of what you don't!).