The name of the candy M & M stands for Mars and Murrie. Forest Mars was the founder of the Mars candy company; Bruce Murrie was the son of the president of Hershey's. The "m" stands for each of their last names.
The character M&M is Cathy's brother. His name is more ironic than symbolic. He's "addicted" to M & M candies the way others in the book are genuinely addicted to drugs.
M&M the candy was named for it's inventor and a stock holder.
The inventor, Forrest Mars, Sr., was the founder of the Mars Company. The other M was for Bruce Murrie, the son of the Hershey's president who had a 20% stake in the endeavor.