The 4 oldest sisters are all in high school, I read Lynn is in the 8th grade and Lori is a senior, revealed in "Linc or Swim". I figure their birthdays are late in the school year or over the summer.
According to the Episode Party Down, Lori was going to throw a sophisticated party for the older kids including Leni, Luna, and Luan but at the end of the Episode, Lori realized they threw a New Year's Eve party with Lori's friends