Yes, it's true. Izzy's older sister, Alicia, once convinced her that she was a robot. It happened when Izzy was only 9 years old. Alicia told her that she should stop being such annoying brat or she would be replaced. Just like the last one. Izzy didn't really know what Alicia meant by this. Alicia explained that there had been at least 3 Izzys so far. The original, who died a year and a half earlier (think about it, there's no way anyone could've survived that encounter with Bite Mark). And there's been 2 defective robots meant to to take the original's place.

According to Alicia, the first robot ate too much and the second robot wouldn't shut up. The current Izzy eats too much and won't shut up. Izzy obviously didn't get it. She had memories back to when she was really little. She had definitely been alive longer than a few months. But Alicia went on to say that Izzy's memories were implanted. What about the parts of Izzy that were flesh and blood? Sophisticated robot parts. Parts that were so expensive their parents had to cheap out and use regular robot parts for the rest. In fact Izzy's memories had to be altered so she wouldn't remember how rich they had been before the family started getting replacement Izzys. The next one would probably be even cheaper than current Izzy. 

"One more thing. If they find out you know you're a robot, they'll replace you even quicker," Alicia said. 

This scared the crap out of Izzy. For at least a month she tried desperately to keep from being replaced. She talked less and ate less. When her parents asked her what's wrong, she usually said everything was fine. Until one night after being asked one too many times,  a frightened Izzy blurted out "Please, don't replace me!"

Ted and Madeleine had a long talk with Alicia. She was grounded for 3 weeks. 

Over the years, Izzy was able to find humor in the story. She can't believe she was so gullible.  She's told it a number of times. She's much more eager to tell that story than the story of her 8th birthday. Maybe the crew won't hear it today, but they'll definitely hear it soon.