I can see the half sister theory. Didn't he already date a girl that ended up being roomates with her? I thought there was a reference to the roomate being in college and I think that was before we even knew about the half sister.
Or they would really piss people off and when the series ends we see the guy from Full House talking to his shrink and we find that he has was a delusional schizophrenic the entire time and all we were seeing was what he thought happened as he tells it to his shrink. That might actually be one awesome ending.
It would explain why the elder Ted somehow has a different voice than the current adult version.