What can I do to fix my inward-turning feet?

yes it probably was a habit you picked up or a condition you both inherited. it wold be easier to fix now than when your older, but still you have done alot of growing already and so may be difficult to fix. it only sounds like a cosmetic problem and not a medical problem so no need to worry about your health, but go see an orthopediatrician and they will be abel to show you your options, they may gives you braces for your legs or at the worst have an operation to pin your legs straight , but that is highly unlikey becuase you problem is not medical.