Is the Hadd punishment for zina applicable to a girl who committed zina with her brother when she was eight years old and unaware of her action, and is her repentance accepted?

The girl is not held accountable for what happened because the pen is lifted from her before puberty. The ruling of zina does not apply to her, nor is she required to repent. She should remind her brother to repent if he was an adult at that time, either directly or by gifting him a tape or booklet about repentance. Both of them must be warned against khalwa (being alone together) because Satan might remind them of what passed.