I am about to go insane over here....

Hello, I am 20 years old going to college for fashion merchandising and minoring in dance. I am currently a sophomore in college. Every weekend, I come home to babysit for my mom. She has three other kids from a different dad. There 2,5, and 8. The 8 year old has a disability. He can't walk, eat, see, or play like a normal child his age. I have to mix medicine, put him on his different machines like, every hour. I am constantly on my feet. While my mom is at work, I have to babysit all three of these kids. But I am not by myself since my step brother (who is 18) sleeps in all day and only comes out when my mom comes home. I get no help from him at all. He is originally from florida but came to live with us a year ago so he can save money for his unborn child. Well, the whole time he has been here, he only sent $150 for his baby and the rest is for himself. He bought a $300 xbox1 system for himself. He also goes out and drinks and gets high. It pisses me off so much since he is nothing but a loiterer. He doesn't help clean or offer money to help out. He only washes dishes and does laundry when my mom comes home. He also treats the five year old terribly by calling him names. My mom goes to my step dad and tells him about all of this so now, he isnt allowed to be around the five year old. I understand that but, I NEED HELP. I am so exhausted and it is just so unfair. My mom is literally all bark but no bite. She complains to my stepdad about his son and how he needs to leave. She even gives him an expiration date on when he needs to go. And if i am correct, his expiration date ended 6 months ago. ANd he is still fucking here. So now he has another expiration date and it is in 2 months. My mom says he needs to save up money for a plane ticket to go back to florida. ANd in all honesty, i dont believe it. I want to say fuck it all and tell my mom i will come back to babysit only when he leaves but, then i feel bad. Idk what to do anymore. We do have nurses for the disabled child but they only come a couple times during the week when i am school. I just need a break from everything.... i have been doing this for 8 years, never had a ''teen" life because I am takimg care of my moms kids. And when i dont want to babysit, she gets mad and says i only think about myself. I am lucky enough to see my boyfriend twice a month. Ughhhhh i sometimes wish i can pull a trigger thru my head )':