The real Laura Ingalls Wilder did have a younger brother, although he passed away when he was less than a year old. Her brother was called “Freddie” or “Freddy”, but his full name was Charles Frederick Ingalls Jr., named after his father.
Freddie died at ten months of age, although the exact cause of death is unknown. However, many babies and young children of the era passed away without the specific cause being known.
Read Charles “Freddie” Ingalls Jr Biography for more on Laura’s brother.