Does a girl bear sin for engaging in marital acts with her sister at the age of eight unknowingly, and what must she do to purify herself from this sin now after remembering those acts?

There is no blame for what was done before puberty, because the Prophet, peace and blessings be upon him, said: "The pen has been lifted from three: from the child until he reaches puberty, from the sleeper until he awakens, and from the insane until he regains his sanity." If the sister continues to do that, then she must be advised and shown the dangers and consequences of her action.