Q.
A. Girl found out through Dream Guy that Sister killed Mother. Girl killed Sister to avenge Mother's death. Sister killed Mother because she found out something terrible/horrible about Mother that she could not forgive her for. Dream Guy knew that Sister killed Mother because he and/or his friends and/or relatives were somehow involed in Mother's past. He either went to the funeral so that he could tell Girl about what he knew (because he wanted to set the record straight, or he wanted to warn Girl of eminent danger) or for some other reason relating to his connection to Mother. The fact that Dream Guy was Girl's dream buy is purely coincidental.

Uh... do I test clean?