If i repeat year 12, what will my chances of getting into an ivy league school in america be?

i'm australian, and it has always been my dream to study at harvard college. my father, who died when i was 5, was american and he studied there and my entire life i've wanted to study there too. i'm 20 years old and completed year 12 three years ago but i graduated with average results (an enter score of 75), there have been quite drastic things that have caused me to graduate with these results and i have been forced to work since leaving school in order to support my family. my mother was admitted into hospital for severe depression and attempting suicide several times, and she was unable to work. my sister is autistic and also cannot work so for the past three/four years i have been working to support my family. my mum has finally recovered some and has retuned to her job as a nurse, and i feel like it's my turn to finally achieve my dreams. ok i'm terribly sorry for the sob story but i just want to explain my situation. i am wanting to repeat my final year of high school at tafe, in order to increase my chances of getting into harvard. i know it's tremendously competitive, so i'm just wondering if repeating my senior year would hinder my chances of getting in? i know they also consider community involvement and extra circular activities and i am willing to work very hard to achieve these. so given my age and my situation do you think it would be at all possible for me to get in? thank you!