Original Commentary

We considered doing this a few different ways - Brisbane deciding to wait longer, or Sofia and Brisbane going together. Waiting further would allow for more personal growth, but we decided the plot needed to move forward. There wasn't anything pressing to do that involved them being separated. Switching to a B-plot involving other characters would be gratuitous - Terrence

Modern Commentary
The other advantage of Sofia not coming along is that she doesn't find out this is about sex. Sofia doesn't want to know anything about her brother's sex life and vice versa. - Terrence