Guide to Fancharacters by ~Fullmetal-Phantom
If a family history is known, it is set.
Suppose the main character is known to have a mother who visits occasionally, a father who is dead, and a younger sister who is a regular character. Your character is not his long-lost twin, because he has no long-lost twin - you would think that he, or at least his mother, would be aware of another child, now wouldn't they? Attempting this causes your character to feel shoehorned in, and it won't leave your viewers particularly pleased.
Ahahahaha. I don't have to create a long lost twin brother because DATA ALREADY HAS ONE.