Chlorina Bouvier or maybe Bouvier-Jones (née Jones) is the wife of Seymour Bouvier and is the mother of Moonstar Bouvier-Jones. She is Marge's cousin and the aunt of Simpson Family and the cousin in law to Homer Simpson.
In many cultures it is traditional for the wife to take her husband's surname when she marries, and any children born would also have the husband's surname.
However, in The Simpsons Uncensored Family Album, Seymour is shown as having the surname Bouvier. This does not appear to be a middle name and it is unknown what circumstances led to Seymour having a different last name than his father.