Maybe Petunia does know that they are related, but doesn't admit it, becasue they arent' 'respectable' enough... She wouldtn' want people to know that she was related to the Potters after all.
NOt saying that hey are a family of wizards, but maybe they have different ideas/values to the Dursleys, ie, maybe htere is actually some fun in their lives! They might do stuff like actually go on hoildays, where they LEAVE THE GRASS UNMOWED!!! AHHHHHHHHHHHHH!!! Petunia certainly woudln't admit to being part of that family!!!
I like the idea of his parents being like Hermoine's though, it makes alot of sense! And his age is wayyyyyyy to coincidental!