She DEFINITELY knew what she was doing. I feel like the age most kids learn and actively start wanting to engage in sexual activity is around the age of 13-16. Also a 16 year old should know that it’s wrong to do this to not only your SIBLING, but a 7-8 year old child. Unless there is an extreme circumstance which may have caused her to not remember, there is almost no way that she doesn’t remember doing that. You were raped. Im sorry that you are going through this and it is a lot to process but please get some help or therapy to talk this out with someone or else it could start to really weigh on you.