A lot of children have to have their elder sibling's hand-me-downs, even if the elder sibling is a different gender. But, since Paul's big sister hadn't grown out of her clothes yet, his mother says he'll have to stay in his own clothes. His baby clothes, actually.
"I bet you regret playing Truth or Dare with me now, don't you baby brother? But then again, it's a lot of fun to see you squirm now that Kathy is involved, especially considering the crush you've had on her for a while now. I'm sure she'll make a great babysitter."