What does it mean if you dream about maggots in a family member's old house?

Maggots represent rot and decay, and most people react to maggots with revulsion and disgust. The family member's old house would symbolize that person's past, perhaps their family history. However, your dreams are about YOU, and your family member's family history is necessarily your own family history. This suggests that the dream expresses feelings of disgust and rejection that you feel toward something in your background, heritage or ancestry. By setting the dream in the context of another family member, your mind is (symbolically) distancing yourself from that history.