This wiki states that John was adopted at birth, and do not know his biological parents. Is there anything else than what is seen in the episodes to back this up?
5x03 does imply that he had an adoptive mother, but not that he was adopted at birth. For all we know, both his biological parents died, and he was adopted after that; moreover, when John goes back to Puyallup (3x11 or 12, not sure), Fusco recognize Reese's father on a photo, from their similar looks, on top of the fact that he's wearing his uniform.
Call me stupid if you want, but what are the odds that an adopted child looks like his adoptive father?
(John's mother could have died in childbirth, or left, too, and John's stepmother could have legally adopted him when she married his father, for example.)
(Or John's mother might have died after his father, for some reason, accidental, health-related, or even depression, and arelative could have taken John in, adopted him. That happens, or am I wrong?)
I'll just add that Kara states he never really had a family, just before talking about his adoptive mother, but I wouldn't bet on the exactitude of her intel, given that itmost likely came from John, and the guy has made an habit not to be very truthful. After all, the Machine lists him as having a sister, which doesn't fit, again, with Kara's claims.
And even if she did know, she could simply be meaning that he's estranged from his sister ( could be way older than him, or younger, what do I know ), and that given his father's service, he hasn't seen him much.
Personally I tend to think it's all a continuity error, but I'm pretty sure we can make something that actually works out of it, rather than deciding that one statement from Kara negates everything else we saw in the show. What do you think?