During an interview to promote The Long Run Glenn mentioned that he had an 18 year old brother who listened to the kind of music that the Eagles were experimenting with on the album (Teenage Jail, Disco Strangler). That would make Matt 13 years younger than Glenn.
I have heard Glenn say that he put his brother through college, but I'm not sure that this is where I heard it.
Soda, Dreamer & PLS heard Glenn talk about his mother getting married for a second time during one of his concerts in Niagara Falls. If I remember correctly, he said that his mother remarried when he was three. Please correct me if I'm wrong ladies. Alan is five years younger than Glenn so he would have to be from their mother's second marriage.