Should I have to pay rent when living with my mom?

My mother makes an exceptional amount of money but she claims she makes way less (which is not true). She's asking me to pay rent but I'm pregnant, going to school and have a full time job (bf doesn't live with me). I also have an 18 yr old brother who doesn't work, skips school (high school) and has stolen money from my mother in the past. I plan to save up & move out but I can't do that if I pay her rent. That money could go to my savings. She refused to ask my brother to get a job and insists that the money come from me. She even demands to see by budget to see if I actually can afford to give her money. I want to tell her respectfully that I don't think it's fair for me to pay when my brother isn't but I wanted a parent's opinion on why she might be acting like this, I feel like I'm just being picked on. BTW, we haven't always gotten along bc she has a reputation on being somewhat of a bully with me.