Straight Dope weighs in:
http://www.straightdope.com/columns/060113.html
http://www.straightdope.com/columns/051209.html
Greek Mythology Family Tree line of Achilles and Patroclus:
Aegina: daughter of the River God Asopus; mother of Menoetius by Actor and mother of Aeacus by Zeus.
Menoetius mortal King of Opus, son of Actor & Aegina; father of Patroclus.
Aeacus immortal King of Aegina, son of Zeus & Aegina; father Peleus who was in turn the father of Achilles.
So based on our Greek Mythology Family Tree, Patroclus was 1st cousins with Achilles' father, Peleus, making Achilles and Patroclus either 2nd cousins, 1st cousins once removed or uncle(Patroclus) and nephew(Achilles). In other words Achilles and Patroclus have the same grandmother(Patroclus) and great-grandmother(Achilles), hence the reason why ancient Greeks made Patroclus older of the two and since as it was pointed out not even that idiot Dover who is credited as the "all know how" on ancient Greek sexuality, NOT, does not state anything about male on male incestuous relationships in his book. ;)