From what I have read, we don't really know if PG and ALJr are common-law, or married, or how long they've been together, and I don't recall ever seeing whether any of their children are from a previous relationship or whether any of their children are from 'their' relationship. Has that been stated somewhere?
From photos I've seen, he appeared to be about 12 when Nathan's parents were married, so he should be about 16 years old today. That means that he was born in roughly 1998, when Douglas was a fugitive. I think it's a fairly good guess that Allen and his wife met and started a family well before Douglas met Alvin.