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It is because your father's case and your mother's case are irrelevant each other, i.e. the two cases are independent (your mother is not the dependent of your father to get the green card, vice versa), you cannot check "2".  But if you sponsored your brother's green card, then his wife and his kids can get the green card as his dependents/derivatives, thus you need to check "2" when you file I-864. But in your parents' cases, you need to file two I-130, and two I-864s because your father and mother are not dependent each other.