What is the relationship of Clodia to Quintus Caecilius Metellus (Pius)?

Clodia's relationship with Quintus Caecilius Metellus (Pius) is that she was indeed his wife. This relationship is significant in the context of Roman society, where marriages often served political and social purposes. Clodia was known to be a member of the prominent Claudian family, and her marriage to Metellus helped solidify political alliances.

In examining the other possibilities, she is not his sister, as historical accounts do not indicate such a familial connection. Similarly, she is not his cousin; their relationship was one of husband and wife rather than that of more distant relatives. Lastly, she is certainly not his daughter; their life together as a married couple is well-documented in historical texts, which disallows any notions of a father-daughter dynamic. Thus, her role as the wife of Quintus Caecilius Metellus (Pius) is the correct and most appropriate relationship in this context.