Oh, right. Need to remember to interpret the spell correctly.
Wrong.
Correct, Girard's grandfather was a blood relative of ABD.
Wrong. Since Girard's grandfather was a blood relative of ABD, Girard is also a blood relative of ABD. If your brother is your blood relative, your brother's grandson is as well, as you share a bloodline. The same goes for the other Draketooths. Hence all Draketooths were killed as blood relatives of ABD.
Wrong. Since Penelope's child was killed as a blood relative of ABD under Clause 1 of Familicide, Penelope was killed as a blood relative of an ABD relative under Clause 2 of Familicide. Penelope's family, those who share her blood (and her child's), would also fall under Clause 2--including a hypothetical child by Tarquin. Nowhere is a third iteration required, let alone a fourth.
Wrong. Your cousin is your blood relative; hence the same step of Familicide that kills you kills your cousin; hence no additional step is required to kill your cousin, or any of your other blood relatives.
Good thing it's limited, then.