The preposition to use before "uncle" and "aunt" in English is "of." This is the most common and widely accepted preposition when referring to one's relatives. For example, "my uncle of the Smith family" or "aunt of the Johnsons." The use of "of" is based on the traditional structure of English when describing relationships, where the preposition "of" is used to indicate membership or association with a group or family.

This convention is supported by the American Heritage Dictionary, which states that "of" is used to denote "membership in a family or group." Additionally, the Cambridge Dictionary echoes this usage, suggesting that "of" is the correct preposition to use when talking about someone's family relationship. The use of "of" in this context is not only grammatically correct but also reflects the standard usage in English-speaking communities.

Data from the Corpus of Contemporary American English (COCA) further reinforces the prevalence of "of" before "uncle" and "aunt." In the COCA, which is a large database of American English, the phrase "uncle of" appears significantly more frequently than any other preposition followed by "uncle." Similarly, "aunt of" is the most common phrase when discussing an aunt's relationship to a family. This data indicates that the use of "of" is not only correct but also the most common and accepted way to express these relationships in contemporary American English.