I hate to be the bringer of bad news, but, your sister is right. You have Type 1 diabetes. Short of several major medical breakthroughs, your pancreas isn't going to start working again. EVER. This is reality. Doesn't make it easy to accept, I know... but take a look at the list of complications that come from poor blood glucose control whenever you get the urge to think "I don't have this disease anymore." You can tell yourself that's true all you want, but you're still going to suffer the consequences in the long run.
There can be all kinds of reasons your blood sugar didn't go higher. Maybe you're coming down with a bug and your immune system is working overtime. Maybe the training required so much brain activity that you burned off a lot more glucose than you expected (the brain doesn't need insulin to take up glucose, so intense concentration CAN make you low). But I promise you, your pancreas did not magically rev up a whole bunch of new beta cells!