Keeping It In The Family

While at dinner one night...
     Brat Child: Can I marry Midge?
     Me: No, you can't marry your sister.
     BC: Not even when I'm as old as you?
     Me: You can never marry your sister even if you're old. The law says you can't.
     BC: But what happens when I get old and she gets old and she has another family. I'll miss her!