What does it mean when dream a horse in my sister's house?

If the dreamer is female, the sister is probably a stand-in, representing the dreamer's own self. The horse in the house represents a secret that is not really secret, something that effects everyone in the family, but is never mentioned. For example, it might be common knowledge that "Uncle Ernie" is an alcoholic, but no one ever refers to it more directly than, "Uncle Ernie's problem." Until the family is willing to admit and address the "secret" problem, it cannot be resolved.