My dads girlfriend is slowly tearing my family apart

I am 18 and my parents divorced when I started high school. I am now in my first year of college and in four years my family story got really complicated. My mom was unhappy with my dad so she divorced. My dad still loves my mom and I think he is just lonley. My dad is 59 and my mom is 50. I now live with both my mom and dad in the same city so the commute is not bad but the constant switch is hard on my sister and I. My sister is 20. We both respect our parents decision to split up but it happend when we were aware of what's going on. Most of my friends parents divorced when they were young so it was easier on them. I handled the divorce very upset. It a acted as a catlyist for my high school drama but all that is past me. High school is another story in itself. My sister reacted to the divorce like any other kid. She took it hard when it happend but now she just avoids talking about anything emotional. It's almost like there is an emotional barrier between us but other than that we are best friends and we rely on eachother. We always have eachother to turn to. Since my mom got the divorce my dad always had a hard time adjusting to a different life without her. My parents had been together for about 20 years I think and watching 20 years of someone's life disappear like that was hard to witness. My dad started to concentrate on his job more than me and my sister. My mom was very supportive and attentive throught the process. I soon learned that whatever I told my mom went straight into dads email and whatever I told my dad, he just kept up inside. He always said stuff like "typical mom" just to prove that he is doing better because of the divorce. Now 4 years passed and my parents start dating other people. They are still not divorced to make financial things easier. But my mom could never find a relationship to last. They would just be flings but nothing quite serious. But she soon found out that she needs to take it slow. My dad on the other hand has a obsession with African American woman. I have no prejudice, I just wanted to be specific. My parents meet thier partners on match or some other site. My dad met a woman and I won't use her real name. Ill call her Rala. My dad invited this woman over for dinner with my sister and I so we could meet her since they had been talking for a while. My dad is not much of a cook so she cooked dinner. We didn't know what to expect but when we met her she seemed nice. Rala was smart and polite and my sister and I liked her. She cooked chicken with rice and couscous. She is from Africa and came to America about 10 years ago. We soon figured out we have to eat her way. If you don't finish what is on your plate she will give you more and respectfully give you more and more. I asked her why and she used statements like "where I am from" and "in my country". I ate the extra food and went back to my room. About a month later she started to stay at our house with my dad. She would stay there more often about 3-4 times a week. My sister and I grew a little bit uncomfortable when she would cook our meals and stay with our dad more often. We learned that she had a previous marriage. Rala claims she had married a millionaire but he cheated and didn't want any of the alamony in return. She has a son and a daughter. The son stays with her and the dad and the daughter only stays with the dad which I grew suspicious on. So she is coming to our house nearly without any money to support herself or her family. All the while my dad dosen't care about what my sister and I have to say and she stays for holidays and such. We have a family reunion every year on my dads side. My dad sister and I went and it was fun. Rala didn't come because my dad didn't invite her. I thought that if she is spending holidays with us then she will go with us on this trip. But my dad was almost emabrrased to bring her beacause and I quote, "the family would be shocked", my dad said. I did t question it because my sister and I were growing not to like her calling my dad four times a day, looking through my dads messages and a answering our house phone as if she was living there. She might have well been living with us. I soon learned that my dad was paying for Ralas nursing school, gas, car, schooling and rent. She is a nurse and has crazy work hours. For example she would start work at 8:00pm and end at 6:00am. She always goes into our house shuffling her way through. She always sais Hi and then rushes to my dads room, locking the door behind her. She started to notice my sister and I avoiding her because we learned that my dad was slowly going broke because of her. I soon learned that my dad would low run on money right before his paycheck comes and would take money from my account to pay for her schooling, her gas, her food that she should pay herself. My mom often texts and calls my dad to discuss family stuff like money and soccer practice. Rala would go through my dads phone and delete the messages from her and pick up the phone when she calls. This is totally unbelievable and I grew so angry. Rala brings her son to our house too and we let him play video games and he is a good kid. I feel bad for him because his mother can support him without us. And my sister and I talked yesterday and we both agreed that if my dad broke up with Rala then she would have no money, no school. So even if they get in an argument, she is going to do whatever she can do to stay with my dad. I have many reasons to be upset at her but one is beacause of her implementation of her values in my dads house. She shouldn't have any authority until she gives me respect. She haven't got to know either my sister or I and we want her out. Today I learned that after a year my dad wanted to break up with Rala. She had called the house 5 times and I ignored the phone after the second time. I went to school and my sister took me to my moms because she didn't want to hear them argue. I wanted to get then to break up because she has become an expense on our family. She is ruining the relationship between me and my dad and I can't go on like this going into my room whenever she is over. I am hoping they break up but then I remember my dad is the only reason she has a house so she is going to be desperate and do anything to stay with my dad and his wallet. My dad is going to retire soon so do you think that she will leave when the money is gone and we are broke? What would you do if you were in my situation because I am only 18 and can't move out. I need advice.