When Bart confesses his desire for a younger brother, and Marge explains that she and Homer are fine with just three kids, Marge points out to her son that even if she and Homer do end up having a fourth child, the baby may end up being another girl instead of a boy, resulting in Bart having three sisters. Shocked by this realization, Bart imagined himself as a teenager, being forced to go shopping with an older Lisa and Maggie, and an older version of a possible third sister.
Lisa, Maggie and their sister forced their older brother to carry boxes and bags of clothes they had bought, and listen to their talking, in which Lisa claims that, "Shopping is the most fun you can have standing up." When the vision ended, Bart realized that he couldn't risk possibly having another sister.
The Simpson sister's only line was, "You didn't see what I cooked up in the last galley on my flight to London. They don't call it 'Virgin Airlines' any more," resulting in her sisters laughing.