PLEASE HELP ME DO GOOD ON MY EXAMS, PLEASE?!!?

I really want to get A*'s in my GCSE (exams), my mother is sick, my father is at work all day...I take care of MYSELF literally. I live with my bro and my 2 younger sis. I'm in charge of the house and everything I can't handle it anymore...I'm so stressed out, my exams are next week literally. I'm in year 10! And I want to pass with A's to make my family proud....I revise hard, I try hard in school yet I need that something which will boost me up to A's because right now I am a B/C student. Please help, any ideas on how to get A's in my exams....I have just three days left to prepare until my FIRST exam...urgh I feel so stupid, I always start to cry and feel so incapable. My mind tells me that I am capable but I just need help...please help me! What do I do? I revise a LOT. I revise and concetrate in class, but I can't seem to get A's Or A*'s, I use up half of my day praying for my mum, looking after my baby cousin (her mum is also in a weak condition), I run up and down the stairs cleaning the rooms to please my mum, I pick up the mess when my 14 and 12 year old sisters just throw chocolate wrappers and stuff on the floor, I mop and brush the floor , I go and put my brother's back and shoes, and sort his bag out according to his college timetable, I do all this and i'm younger than him! It's just that hes such a lazy bum, but I respect him alot, I respect my family and I believe hard work goes on to success. The dinner (my grandma makes and gets my cousins to deliver it to our house) then I do the washing up,drop my baby cousin off home...and then after everything is done my dad comes home from work...and I have to carry his suitcase all the way to his room upstairs, and THEN I get my free revision time! What do I do!! I'm so depressed :'(.