I agree. You were a young child and didn't have the sense or impulse control that you have now. In a very literal sense you are a different person. The best way you can make it back to your sister is by being the best sister you can possible be.
As for hell, the idea of sending anyone to hell for something they did as a kid just seems so wrong to me. But, perhaps more importantly, it is very unlikely to exist. The idea of sentencing someone to eternal punishment for finite behaviour makes no sense... remember, your time on Earth is the smallest smallest fraction of a second compared to infinity. It would be like marking an exam paper 'F-' based on the way you picked up your pencil before you even started writing!
I still find the idea of hell scary as a guy in my late 20s... but I can also see it's obviously invented by humans. People don't like the fact that life is unfair and that sometimes bad things happen to wonderful people and other people can so horrible things and reap no consequence. People comfort themselves with the thought of their enemies suffering forever while they look on happily from heaven. But since this world is messy and chaotic and not organised as 'all good' or 'all evil' I don't think it is likely that any place after this would be organised like that either.
In short, you don't deserve hell or eternal punishment (which isn't going to happen anyway). You are just punishing yourself and your sister will be best served by you putting the past behind you and treating her now with kindness and happiness

Good luck and all the best!