This is no joke, Nikki. I would REALLY recommend to call Child Services. Tell them your mother is not fit in her position, and explain to them what you stated here.
Is your brother older than you? Because if he isn't, and you turn eighteen, that's kidnapping.
Have a private meeting with your father and brother about your mother. Discuss what she has done, and what her habits are. At some point you are going to need to confront her. Especially about the cheating part; If you can pin that on her, she will get antsy.
Tell your dad to not give her any more money, and that she needs to get a job.
Hope that helps ~ Benny