Why isn't Dakin Williams portrayed in The Glass Menagerie?

I’m doing an IOP in school, (Individual oral presentation) and its a big part of my grade…
The topic I chose is ?How does Tennessee Williams ?The Glass Menagerie? mirror his own life?
I was trying to find information as to why his younger brother Dakin, isn?t portrayed in the book, but can?t find any. Anyone know anything about it?